Admins already has the ability to lock threads - though it doesn’t happen that often. Original posters, however, does not have that ability AFAIK.
News items sometimes spark a debate for good reasons, and I don’t see any particular reason why that should be limited to single posts, or discussions should be forced into a separate threads.
I agree that posters should strive to use the appropriate channel for discussion, and one could argue that this OP should have gone into Site Feedback, which is “Discussion about this site, its organization, how it works, and how we can improve it.” while Feature Requests is more for new features in Vassal proper - see also Categories.
The one place I think it may make sense to limit replies would be in the Documentation category. Since that forum category has taken the place of the older Wiki documentation pages, and thus serve as user documentation, I fear that long threads in that category could potentially be confusing to users. However, there’s something to be said for keeping the discussion close to the documentation so that users do not need to go look through the forum.
Yours,
Christian