A mechanism could be devised whereby threads could self-lock by default after x days without any activity (i.e. 90), giving the OP the option to adjust or even remove the default value x.
This could be implemented, but would do little to improve the forum, imo.
The way I see it, threads which have little activity is not a problem and usuaythey just fizzle down to several pages down and are not seen by majority of the forum. Even if they are bumped, they most likely sink to the bottom soon after.
The spam problem comes from threads which are still being actively discussed daily with no actual contribution. Replies here are just remakes of the ones before and those commenting d not even bother to read previous, replies and surely cannot read 4-20 pages of posts.
If an autolock feature is added, it would d nothing to reduce spam mega threads.