[CarlitoBrigante]

In all of our clients' websites, we usually setup some of the methods in this thread. But since a lot of spammers these days are real people, the best defense should include also a usergroup promotion system.

Basically, the default usergroup people get at registration offers limited use of PMs, no links in posts, no images, and so on. You then establish some criteria - number of posts, days registered, etc. - for promotion to the next usergroup. This discourages a lot of spammers, also on very popular forums (we used a setup like this on some of the biggest vB forums).