UBB.threads PHP Forum Software Community Forums Resources and Customizing Admin Zone Integrate - for Registration ( http://www.stopforumspam.com/ )

They have an API, shouldn't be too much work to check IP/Email etc live against their list.

But somehow, we hardly ever (maybe once or twice in our existence) get any spammers. But then again, we kinda check names, and your profile has to be filled in with some sort of logic information.