How about automatically closing off a thread if nobody has posted to it for, say 2 weeks? Lately, there have been some newbies bringing back dead threads from months ago.
The expired threads would not look like a thread locked by a moderator with a line through its subject. You just won't be able to post to an expired thread.
Here's what it looks like on RoadFly, but they use different forum software. You get the idea...