Its never going to work on the suing stuff, but Rambo your idea was by far the best so far on registering CDKey to be able to login and play online.
Right now the only two things that will help against this are:
Admins: A good one
Passworded servers: Enough of the old school here to advise a way of getting together on someones forum to pass along passwords to each other, only works if you keep changing it though.