Do you allow guests to post? Your first line of defense should be to turn that feature off and never even consider it again if you are currently allowing it. There's no need for it, especially on a forum.

If you already have this disabled, enable user account activation next. Some spammers are sophisticated enough to activate accounts, but this will reduce your load considerably.

The last and ultimate line of defense against spammers is disable your forums. Other than that, you should employ a respectable group of moderators to help you eliminate bots that slip through the cracks, because no matter how many layers of defense you implement, some will always get through. As a forum administrator you need to be proactive, and utilize as much preventative maintenance as you can without becoming a hassle to your legit members.