Board Game Arena

Undo should be disabled if a user is out of time. You can't kick the user since they are still making moves. Make a move, undo, move, undo, repeat. Run the clock down and there is nothing you can do.

Or, remove the 20 second wait from the kick button.

I think if I'd face this type of behaviour, I'd just quit that game. Karma points are very easy to be recovered soon.

Mutual abandon works as soon as the opponent is at -0 sec, afaik, giving his permission by default.