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wow. i am really sorry to hear that. no, you can't retract. but you could try and play it off as a joke?? :(
Tell to him:"Don't read my last message". 
I do not believe retraction is possible, but you could think up an alternative plan if he bases his moves on what is in the message box.
lol, i have thought about how many people must make that mistake. Just console yourself with the fact that nothing on a chess board is hidden, and be extra careful not to make any blunders or weak moves that you've told him about. if you want to be sneaky send another "message" about a continuation for if he plays a weak move, and how you will make a subtle but losing blunder if he does
And, of course, your opponent will never think to READ THE PUBLIC FORUMS, in which we have loudly given you advice on how to fool him :)
I wrote my notes on a game in the message box and sent it!! Now my opponent knows my moves. does anyone know if I can retract it? Quickly before he's back online