My advise to you is to remove any mods that "prevent spam" as "I" personally believe that keywords within these mods is what actually attracts these spammers whether they are human or not.
I do not use any mods to prevent spam except this mod here (
Ban Spiders by User Agent) that helps with all those bots, spiders and crawlers eating up bandwidth.
I also use a very good .htaccess code that completely eliminated those spammers from registering and posting on our forums.
If you need help with this send me a PM Buddy