Spammers aren't really that smart, they're just extremely persistent. I hardly update my various blogs and while they hold decent page ranks I don't really care if they get indexed or not, but they still get pounded on a regular basis.

You might want to try the phpBB3 forums and see what other forum administrators are doing there. We could help you write a PHP script to catalog all your spam hits and pillage the occupants of the servers they bounced off of (and occasionally the spammer themselves when they don't do a good job of forging their headers) but for phpBB specific topics like this you're more likely to get specialized help from their own forums.