1. If you just want to to prevent additional posts in a particular folder or discussion, use the Access List to make it read only for all registered users and guests. For version 5.0+ you could also install the "Freeze Discussion" plugin to do this with one click.
2. In current releases, you can click the Edit Folder button to set the archiving routine for a particular folder. This causes discussions that haven't had any new posts for an interval (set by you) to be either deleted or moved to another location.
NOTE: Expiration settings
do not propagate to nested folders -- each folder stands alone. If a folder does not have any expiration settings, then discussions in that folder will remain until they are explicitly deleted.
3. Or (and also for earlier releases) you can Export a folder or discussion and then delete the original. The export file can be saved for future import if necessary.