[y2ksw]

As a very first step, protect you forums by a good captcha. One good solution to the standard is to add a few TrueType fonts which are difficult to interpret from both OCR and tired human assisted registrants. "Baby Blocks" for example is a really good font, but also "Elevator".

Secondly consider using the question and answer system with some well studied reply patterns. Very few bots do understand how to insert the first letter of the second word all uppercase. In fact they never did.

Third, make one of the "contacts" options selected and sound like: "I'm a spammer and I spam you." Nobody who really wants to contact you will ever leave this option on. But spambots will. That idea came from this forum and it works very well.

Last not least, don't allow guest posts. If you do, enable the same checking system as for registering. However if you are massively spammed, consider installing also some plugin. Search this forum for spambot and you'll find some