Can old threads in the forums not be removed or at least blocked from further replies? Even archiving them is usually pointless.
I say removed because the advice in them can be out of date very fast, and some of the threads go back years!
I suggest blocked from further replies if you cannot remove them, because people replying to postings months or years old is just as pointless.
Perhaps make the dates on them more obvious if you cannot do the above, then perhaps people may notice that they are replying to a posting years old, so they may realise they are wasting their time.