When viewing a thread, each message info box contains a link to search for all a users posts. I.e. "Messages: 5342". The search results page is noindex, but the link to it should probably contain rel="nofollow" - it's generating a huge number of search spider hits to a noindex page. I'm getting huge numbers of spiders trying to follow these links for no reason, as there are up to 20 on a thread page.
It's very similar to the "new posts" link which was fixed: