I've created a new Forum feature that gives teachers the option of having discussions auto-deleted after a configurable period of inactivity (e.g. if no new posts have been added to the discussion for 4 weeks):
On the forum page, at the top of the list of discussions, a warning is displayed to users:
This is most likely to be of use for keeping social forums fresh and uncluttered.
If you think you would find this useful and would like to see it integrated into Moodle 2.9, please add your vote to its tracker issue at https://tracker.moodle.org/browse/MDL-49420 (comments also welcome).