Most boards are slow lately and finding people willing to post is always a problem...
Chris, we use a vB plugin that forces members registering to answer a random question before being able to complete registration. I loaded it up with simple math problems like "5 + 8 =" and reversed words. We had a SERIOUS problem before this plugin and virtually nothing since. Been over a year now. :knocks wood:
I'll find the name if you want 
I've heard of plugins like that for almost all types of boards and can be built for other systems as well.