maybe when a user trys to register for TF2, it should be entirely based on their performance of normal TF, by moderators discretion. those who are fairly active posters, who post useful/interesting posts should be allowed to register.
those who are new and want to register for TF2 would have to spend a while posting useful stuff on TF, and no spammer is going to bother to do that, just so they can get one spam post on TF2 before they get banned.