I.e. lock threads after e.g. 30 days without a new post.
One million times this, but with the caveat that it doesn't apply in all boards.
In boards such as Technical Discussion or Technical Support, often older threads get bumped with pertinent new information, or someone who is having a problem revives a thread of a similar problem. These threads are usually fine, because these boards generally aren't spamfests, and it is often better to keep this information in one place rather than having to rehash all the basics whenever a new problem arises.
However, in boards like Bitcoin Discussion, this is needed
badly. Threads are necrobumped with one line nonsense all the time. You can report 5 spam megathreads to be locked, and within 24 hours there are 10 new ones necrobumped to the first page. There are some users in there who I've seen necrobump 10 dead threads themselves in the space of about 20 minutes. These threads obviously then attract 100s of new spam posts within hours. It's just endless. It's a completely unwinnable battle to get on top of the spam when there are >1000 pages of old threads just waiting to be bumped whenever some spammer needs to be paid useless tokens.
I would also appeal to the non-spamming members to please, PLEASE use self-moderated threads and lock your threads once the discussion has run its course.