BBP Auto-Close Topics will automatically close bbPress topics after an admin-specified time period. The topic age is based on topic freshness. Topics are closed on-the-fly (rather than in the database), so, if the you deactivate the plugin, topics that were once closed will be re-opened.
Development of this plugin takes place on GitHub https://github.com/thebrandonallen/bbp-auto-close-topics. This is the preferred venue for support requests, and pull-requests are more than welcome :).
* Note: bbPress must be installed and activated before this plugin will have any effect.