Why not just lock repeat threads only if the old thread is on the 1st page of threads? Even the 2nd page if theres alot of traffic. Does this make sense?
And the only time i use the search feature in a forum is when i don't really want to post anything, but rather to search for particular information. I suggest if anyone has a question to ask, they use the search first to see if it's allready been awnsered.