There is really nothing we can do as far as auto-assign goes. BUT!--- if auto balance puts a player on the other team and they switch back, admin will warn (or kick if its a persistant problem). The best you can do is remind ppl not to stack teams and let the admin put the smack down on those that wont play nice.
PS: Dak, hopefully you will get a trial soon and you can warn the team stackers)