I'd be willing to bet the javascript trash forum that we're supposedly migrating to won't allow that feature without months of changes. 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.
Hmm, is it that hard to take a look when the first post was posted? I am aware many people here just jump to the last post(+/-s) and don't bother to check the date but this is in the first place their fault and I reported them several times in the press section.