Spot this thread:viewtopic.php?f=3&t=310&p=2254#p2254
Guy asking for how he can advertise on the site. Suppose it's David who needs to respond but I can't help wondering if its a spammer strategy to make you think twice about deleting the message since there are links in the signature to outside sites.
A good anti-spam strategy is a blacklist. We just installed one on the erfwiki and it's brilliant. Its an actual blacklist against posts/edits which prevent people from posting if their message contains a URL thats blacklisted.
Beyond that you can remove the incentive for them to spam with the robots.txt file by excluding the forum from search engines like Google.