Ciris: It's a solution for now, until we solve the spam problem. Then, if we find a better one, we'll implement that instead of this :) we're considering a few now, but are going to wait a few days until we can see if spam has cleared up or not before we make a move (to know if this is a "safe option" in terms of spam protection to fall back on).
Unfortunately, I see no reason for the bots to stop trying. The cost of failure on the spammer end is probably minimal.
This might be an okay fall-back in emergencies, but I'd suggest treating the bots like zombies. They are knocking at the door, and they are never going to stop. Come to think of it, some well placed head shots would probably put an end to the spamming problem too...
I hope ya'll can come up with something better to implement.
JMich: Quick suggestion. Make it a daily thread limit instead, with a suggested limit of 1 + purchased games. New users can make 1 thread per day, while older users can make more (assuming they have bought something).
Though that may mean that you'll have a list of spambots that make an account, post a thread, then make a new account. Unsure if that will be better or not, but it does allow legitimate users to create new threads, though at a much lower pace.
We've already been hit by rotating spam accounts in this very attack. I don't think this will work.
