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.
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.
From mprep's
Unofficial Rules thread:
Regarding necro posting, if the necro post is substantial (doesn't break the rule especially rule 1 and 2 (2014/07/21 version)), then it's OK.
The necro bump on that marriage thread was low value spam - I know because I reported it - and so deserved to be deleted. Unfortunately your post was likely collateral damage.