Akismet is a site that bloggers (originally wordpress bloggers) use to eliminate comment spam on their site. If you have ever run a blog, you know that you start getting comment spam about 3 minutes after your blog makes it on Google.
They have begun porting it out to many other applications:
MT
Drupal
phpBB
Blojsom
Geeklog
Nucleus
b2evolution
Movable Type
phorum
The service is basically free unless you are a very large site then it is a whopping $5/month.
My inspiration for this was the fact that all of a sudden I am starting to get posts about intimacy drugs and cell phones. I get these same posts in my blogs, so I decided to try and see if Akismet would work with VB. It worked perfectly and basically ended this type of spam for me.
I think ideally what you want to do is use this for new users, and when a user graduates to a new usergroup, eliminate this check for them.
In the future, I would like to put in the spam reporting feature into this mod so as to add an option to "Report Spam Then Delete", I am just limited in my time right now so it may take a while.