That's called stalling, and no, it's not acceptable - please report them by following the instructions in https://support.chess.com/article/209-how-do-i-report-someone, but also please don't name them in the public forums, as those sorts of allegations aren't helpful either.
Thanks,
David, moderator
Dear chess.com friends,
Tonight, I played against [Removed: please don't name alleged offenders in the public forums]. I had a massive material advantage of a Queen, two bishops and a Rock. Only 3 moves away from checkmate my opponent just stopped playing forcing me to wait 15 minutes for termination of time without any notice. I used the chat to give him a few warnings without any response. Is this acceptable behavior at Chess.com?
Regards,
llojkalla