I use this to prevent spammers from registering.
NoSpam! for vBulletin 3.7 - vBulletin.org Forum

If they dont know about the subject of your site, they wont know the answer to the question. :)
You can use multiple answers so almost anything related will let them register.

I think it works great.!!

Also i only allow people to send a PM, once per 5 min to one person at a time. (unless they are a registered member, or better)