Ouch - yeah, I'm not sure I would provide anonymous hosting service.
They can pay with bitcoin but they sure as hell better provide some proof of their identity before I would let them have access to a system capable of spamming (or scamming) others.
In credit card transaction, we can use the ID to verify the card holder name. However, in BTC case, there is no way to confirm that the ID received is indeed the real user.
Yes, that is a problem for porn sites too. I have some ideas for a solution but I can't start working on it until I get home (I'm on vacation).
You can require credit card for sign up - and then accept bitcoin for payments after the fact.