Just report them: https://www.chess.com/forum/view/general/how-to-contact-support-1
You can also visit their profile, click "More" and then the bottom button in the dropdown menu is "Report." Don't name/shame other members in the public forums though. Reporting is sufficient.
Please help me report [removed] - V39 and toxic person
Username : [removed] - V39
What did I do to him????
[screenshot removed] - V39