I think that people that join cop slots in which they are not whitelisted to use, should automatically be returned to the lobby.
Occasionally when you are on cop you have that annoying squeaker who starts screaming in group channel and you have to tell him to return to the lobby. If they don't want to and want to troll, there isn't a lot you can do about it except report them to an admin. Adding this feature would make things a lot more convenient.