An alternative idea to technical countermeasures is to do it the way the SA forums have doing it for 10+ years. $10 to get in. Appoint moderators who can ban or probate users at will if they post trash or break the rules. If you're banned you shell out another $10 if you want to join again.

(And no, this is not comparable to app.net. The money isn't sufficient, you need the moderators just as much. You need both.)