It can be hard to tell sometimes, but some of these posters are being really obvious, they're bumping threads that were finished ages ago, definitely not innocent, the funny thing is sometimes they've restarted them and others have hopped onto the bandwagon without checking.
I don't think it matters if a conversation continues after it has temporarily ended many months ago as long as what is being discussed is related to the OP of the old thread. There is zero harm in doing this.
Spam has long been an issue for this forum (and I would expect other forums have similar problems). If you go to the last page of the media forum you will find two threads discussing the amount of spam from 2011.
Helping out with spamAdvertisement Spamhere are the two threads.