Personally I use Q&A and easy questions - but such ones that just a human can answer.
I didn't have automatic spam since ~1 year and I allow guest posting in most of my forums.
I use rel='nofollow' for extern links vs. human spammers. Some are stupid enough to post anyways. My moderaters remove those posts ASAP.
My master plan however is to punch these spammers in the face when I meet them in reality.
I install and configure phpBB-SEO (and other mods) for a fair price and very fast. If you are interested simply send me a private message.