Do they all happen to be of the form 64.68.87.*? We seem to be plagued by them here too, they're a search engine spider from I think Yahoo. I'm not sure how to get rid of them (I suspect maybe placing a robots.txt file somewhere in the root folder of the site with specific info which I'm still trying to find out), but Bleys nicely banned that IP address range here, so I expect all they're seing is the You're Banned page or something whenever it tries to crawl these forums

If the IP isn't of that form, it'd be interesting to know what it is though, it still sounds very much like a search engine spider crawling your site for stuff to put in it's database ro something