Typically, the way that works is through IP Forbid, where you basically disallow interaction with the forum from certain IP addresses or ranges. The tricky part is that many of these spammers intentionally use proxied addresses, or spam from areas that make it impossible to do a range IP ban without taking out whole chunks of a country.
It's something our devs are working on making better.
Thx for the reply. I know someone who has their own forum using SMF (Small Machines Forum) software and a mod called httpBl which checks the Project Honey pot database. They say that the mod httpbl has blocked 3075 spammers and bots in about 2 months and the bots and spammers attempting to join their forum has dropped to zero from 5 a day.
Might be something for the devs to look into.