Scott posted an IpBlockingModule a while back and Ryan implemented it at the Dojo. It cut way back on the comment and trackback spam. I've only gotten one this year and that IP was promptly added to the list. Ryan would probably share his Ip list with you, since it seems a majority of them come from a few Ips, this may work for you too.
I'm also using it on my sites to block Ip's that repeatly hit the server looking for vulnerabilities.
http://www.hanselman.com/blog/AnIPAddressBlockingHttpModuleForASPNETIn9Minutes.aspx
Line 67 of his code should actually be: line = line.Trim();
Brian D also just released his code for a Regex trackback spam blocking solution.
http://briandela.com/blog/archive/2005/06/28/606.aspx
His solution does modify some of the .Text files though. I've been looking at it and I think I almost have it modified into a standalone HttpModule.