if they are 2000 and plus on online chess, the chess.com admins run them through a cheat test to see if they cheat. Many don't but some sneaky ones get caught and they get banned.
BIG QUESTION...
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I agree with Elite_Spartan=If any player is caught cheating They should be baned from Chess.com 
Why some people's account are being closed? They are usually good players who achive +2000. Did they cheat? And what's going on with them?