The trouble is bots or spammers won't read it or listen to that warning
I agree, but that wasn't the point I was making. I was pointing out the forum already has the ability to flag up when the most recent post is several months old - it's not lack of functionality that is allowing necrobumps as bluefirecorp_ suggested, but rather there are some cases in which they are appropriate. I should have made that clearer on my previous post. As we are all aware, spammers care not for reading rules and warnings.
The trouble is it's only show to the first user who bumps it and not those who think they're posting legitimately after.