In the "useless tags" thread we've identified several tags that we'd like to avoid, such as design. However nothing prevents users from actually using those blacklisted tags, which creates something of a constant maintainability issue for us. Including a "this is a blacklisted tag" note in the tag wiki (as has been done for best-practices) might help in some cases, but not all. New users are also unlikely to find the meta discussion on undesirable tags.
In chat, Noctrine mentioned to me that a feature for disabling a tag entirely, preventing it from being used, does exist but is not available to "regular" moderators or general users (only to SE superusers or some such) -- I was hoping it might be possible to allow our moderators access to this feature so that we can keep our tag database more meaningful will less effort.