I see Tags have a global setting to be open OR closed across all forums/subforums.  Is it possible to set a given forum/subforum to closed?  My use case is for Classifieds where I want to force members to use only a specific set of tags/prefixes in their topics.

Open/Closed is a global setting only at the present time, unfortunately. You would need to post within the suggestions area if you wish to see something like this appear in the future