I've built up a couple of forums for people and I still work on a couple behind the scenes. Most of the spamers I've delt with come through easy registration where there is no captcha type element for confirmation of human registration required. Most of them are bots, software that just seeks out forums to spam that have no registration hurdles.
I use keycaptcha on the forums I help administor that requires the registrant assemble a picture before their form can be submitted. You might also find there may be a software that uses a known data base for spam bots by email and user names collected from all over the net that will work with your forum software. That way if they have been caught before that can't bother you.
There are a few things you can do to stop them or the majority of them before they can get in.