This weekend was especially bad for spammers over on a forum I help admin...

We finally just disabled new registrations for a while to keep it down...

One thing you could do is not allow the page to load from someone using its IP address. Make the use of the dns name mandatory. Use vhosts and send the ip to blank page. Im pushing my forum group to do this also...

Of coarse you can check this by changing the ip address of the forum and see if the spam subsides for a bit till the bots find it again...