Pretty sure you could just add a query to the existing forum to check the age of the most recent post, and if it's greater than several months; disallow the post. SMF probably wouldn't be that hard to extend.
You are already met with the following warning when trying to necrobump a thread:
Warning: this topic has not been posted in for at least 120 days.
Unless you're sure you want to reply, please consider starting a new topic.
The trouble is bots or spammers won't read it or listen to that warning then everyone else just piles in like flies around shit because another thread has appeared that they can post in. People should be able to see for themselves that the thread has been necrobumped pointlessly and to not post in it or report it though. Maybe we should make people fill in a captcha when they try necrobump threads and maybe that will stop some of the bots at least.
I don't think a blanket ban on necrobumps is appropriate. There are some threads, particularly in Technical Discussion and Technical Support, where users have encountered the same or very similar issues as outlined in old threads. Bumping these threads with new information or solutions is more appropriate than starting a whole new thread and having to rehash all the previous solutions again.
Agreed. There are sometimes when it's valid but most times it's just caused by a bot or sig spammer. Had two idiots yesterday bump a thread about April Fools just to post about how it's their 'favourite time of the year' or something like that (both were obvious alt accounts as well spamming up threads in off topic).