Events: Map initialization conditions: Actions: Pick every player in all players matching condition - matching player slot status = not playing(or w/e it is i forgot) remove all units beloning to picked player
If you want to do it during the middle of a game either use the event that says Player - Leaves or something like that. You could also use (if that doesn't work) a timer that has Timer - Every __ seconds of game time, to do the same thing.