Whitey - 11:07 pm on Jul 14, 2012 (gmt 0)
John Mueller @ Google said: In general, if you remove the page that is being linked to (such as a spammy forum thread) and make sure that it returns a 404/410 HTTP response code, we'll ignore the links to those pages. If these links are primarily pointing at threads that you've removed, then there's no need to move the whole forum to a new URL. If you find that there's a significant number of problematic links that you can't remove which are linking to general parts of your forum (eg the forum homepage), moving the forum to a different URL might be a possibility (but I'd only recommend doing that if you're absolutely sure that these links are causing problems -- we're pretty good at ignoring spammy links). If you choose to move the forum, then StevieD has some good suggestions on how that could be done (make sure that your 404 page is useful to users too). [productforums.google.com...]
It's interesting that he went on to say that those links generally only harm the specific pages they point to and not the whole site.
But I wonder if this is the best solution when you've got great links mixed in with the bad ones, which cannot be easily moved to new pages.