Just a suggestion, I think you should enable users to lock their own threads. Maybe add this option to certain user-groups (VIP, senior member, etc) if it isnt added already.
I've always liked locking my own threads when they're no longer useful. Perfect example would be topics posted in the market section; once everything's sold, lock it. Or to prevent future flames once a topic grows sour.
Guess it might give mods less to do though.