Hope_Fowl - 7:53 pm on Dec 13, 2011 (gmt 0)
OK, back to the original topic.
Maybe you should hide your site behind CloudFlare.com because they block spammers. You do have to tinker with log if you want to track IPs (they have a WordPress plugin).
The free CloudFlare CDN service hides web servers from spambots, so is another possible solution to the original poster's problem of bandwidth consumption.