I've been religiously following Google Webmaster Tools since last several months and have found out that there's nothing much you can do about the errors reported in it. Do you mark them as 'fixed' or just leave them as they are?
If it's a huge range of links, then I block them with robots.txt (using wildcards, for example).
If it's just threads or posts that were removed, then I mark it as fixed - sometimes it goes away, sometimes it doesn't.
