I often see webinar and other date-related announcements listed in the valuable "unsolved" screen space, even after the date is long gone. On a couple of occasions I've asked the poster (usually a SAS staffer) to remove or mark the post as solved - always graciously accomodated.
If a forum topic could be initially posted with an expiration date, that would be great.
The default expiration could be never, as it effectively is now. Or the community could have some other time period set to mark the topic as "read-only" as Rick W suggested, solving a related problem (re-awakening zombie topics).