Evidently I violated the Good Sportsmanship policy for punishing those who refuse to resign.

correct. they are playing to win. It may be a 1/1000 chance of winning and unsporting but they are playing to win.

By the way, merely promoting all knights shows a very small smidgeon of mercy.  Doing the following against a real person might be considered overly cruel by even an experienced arbiter.

https://www.chess.com/game/computer/18447157

They are not stalling and the OP is. Since they are not stalling they are within rules if they keep playing to win even if they would rather the OP checkmates them and stop wasting their time. If the OP keeps playing and they have a chance of winning it is reasonable and within rules to keep playing. The burden is on the OP to end the game and it is reasonable to complain about stalling even if the person in the losing position plays on since they do not have a way to end the game without losing but would prefer that the game did end.

My way to "punish" those who don't resign (as if I'd really care) is to checkmate as soon and clean as possible.

Honestly get 5 queens....

So which is it? Are they playing to win or would rather lose as quickly as possible? You can't have it both ways, period. If they play to win, they should be happy if OP prolongs the game since it gives them a higher chance to win since they believe OP can mess up. If they would rather lose asap they can resign. I can't see how OP is in the wrong just based on logic.

Also the winning player cannot 'stall' by definition. Since stalling is to drag the game so the other player can't win as quickly as they should.

the fact that they think i can screw up right there is the very reason i'm doing it. i know that they are wrong. and it's offensive that after playing an entire game with me they think i'm so dumb that i'm going to stalemate a board like this, when i spent all those moves to set it up exactly like this.

It's funner with knights. It's a subtle jab that many may not realize but those who do it themselves get it. It means this is all I need to beat you. (Or in the case of the horsies, I don't need anything to beat you, you're already dead.)

Like if you have a big battle that comes down to one pawn. Promote it to a rook and beat them, instead of a queen. 

They can be happy if they get the stalemate. They can reasonably complain about stalling if they lose.

Well that's the thing. They have nothing to lose by playing on other than a minute of their time perhaps? And if OP gets some pleasure out of it I mean good for them but do you ever remember a match after 5 minutes?

because they logically had no choice but to keep playing if they wanted to win. The OP on the other hand could have ended the game earlier if they wanted to win. So if they gain no advantage from the OP stalling (ie they lose) than the stalling becomes a negative after the game has finished and they are entitled to complain.

1. Again. 'Stalling' does not apply to the winning player by definition in a game where resignation exists.

2. You know what is better than having an advantage in a game where you play to win? Having a bigger advantage. Promoting pawns is simply increasing your advantage and is a lower variance winning play, and every player is entitled to it. Sure there can be a faster way to end the game but if your opponent's strategy is to wait for blunders, then it is the perfect and logical counter play to increase your advantage by so much that even one or two blunders won't matter.

3. It's one way or the other. Either we acknowledge the losing player has a chance or we don't. If he has a chance in the game, then the winning player is entitled do whatever he can to secure the win by reducing that chance to 0. If he doesn't have a chance then he's wasting everyone's time and everything is fair play. In fact you cant say they have a realistic chance if they watch opponent promote the first couple pawns but can't do anything about it. At that point they are simply stalling as the losing player.

"Someday they'll learn to resign to the great and glorious me! Stop laughing at me!"

Your diagram didn't copy over but you are apparently planning on Nc3+ Ka1, Nb3# and you are leaving the hope that somebody willing to waste time promoting three knights would be careless enough to either first play Nc3 with their king on a1 or first play Nb3 with their king on a2.  It is a very slim but plausible hope that a person prolonging the game will make that oversight.  I've seen similar players rated over 2000 actually pause in the middle of reaching for the wrong knight and the action of reaching takes more time than simple clicking so that plausibility is slightly higher on-line.

it's a mate in 1. qb2#. which they can't escape as all they can do is move back and forth between a1 and a2. they don't know i intend to promote the knights at that point and it's irrelevant once i start because i can mate them any time i feel like it.

maybe players exist that stalemate in that spot - i never have. and i doubt i ever will. 

Wouldn't that be considered abuse? 

Hikaru does this constantly.   If you get banned and not him, that's blatant favoritism.

SHOCKED, SHOCKED I SAY

He's not wrong. And they are not wrong. There is a wide range of chess playing ability out there. Not everyone is a 1600 or 2600. There are plenty of 600s out there. Even someone rated 1200 or 1400 will justifiably have questions about the ability of the opponent. I have seen (more than once) someone struggle and fail to win a KKQ endgame. Some people do NOT have a grasp of even the basics, and will just promote more pawns because they learned it could be done and winning with more pieces is "easier". 

There is a strong likelihood that at least some of your opponent's assume you don't know how to checkmate with the pieces on the board and therefore are not surprised to see you promote pawns. 

Some players are willing to play until checkmate (allowing the opponent to get the killer piece accomplishments).  I know that in the occasional daily game (after I've made my last attempt to squeeze out of a loss) I will simply do conditional moves all the way to the checkmate (normally playing until checkmate takes an extensive amount of time in daily games and this way it is done with the opponent only spending a few minutes)

If a player that is willing to allow the checkmate to occur sees the opponent making other moves then that now becomes an opponent in a group of players where there is a statistical chance that some of those players will mess it up and stalemate.  You may not ever mess it up but some players like you will mess it up and thus the hopelessly lost opponents have just had "hopelessly" removed from the equation (not very much hope, but it is there).  For that matter, if you are interrupted and time out then they also hold the draw in a position they expected to lose.

I can understand wanting to do something you find more interesting than an immediate mate.  It's just that you may not realize how your actions increase the odds that people will play out such situations.

Decades ago back in the high school club I was a normal egotistical adolescent that would try to make games against some of the weaker players interesting by delivering checkmate with the h pawn.  For an adolescent not yet old enough to get a driver's license that may not be admirable but it is not particularly surprising.  Before I graduated from high school I saw no further need to do that.  I've worked hundreds of tournaments and the great majority of those who do unnecessary underpromotions while retaining a mate in one are either adolescents or lower rated players (obviously 100% in the scholastic tournaments I work but still the great percentage in open tournaments where scholastic players make up a minority of the entries). There are still a few higher rating adults that do it out of annoyance (in an in-person tournament there are observers who might make comments about the silliness of playing on but that is much less likely on line).  From a ratings integrity standpoint I prefer to see such piling on rather than those adults who get so annoyed with a non-resigning opponent that they quit in disgust.  At least piling on is not as much of an incentive to continue playing on against an obvious forced mate (still some incentive but nowhere near as much of one) and the occasional quitting in disgust might actually be an accurate component of the player's normal winning chances.

Doing what you do is...something a jerk would do. I do hope it does indeed warrant a repremand.