I just added tag-request as a sort of advisory non-tag:
tag-request
DO NOT USE - Any user with the create-tags privilege can create a new tag. Please leave a comment on the question requesting that someone do so instead of posting the request on meta.
Hopefully when someone sees it they'll follow the instructions therein instead of posting on meta.
A tag suggesting that it not be used feels a bit funny, but there are certainly plenty of questions on meta that are tag creation requests. The answer's always "You don't need to post here," and it'd be nice to not have those questions at all.
If tag-request is used in this advisory way, addtag-request should be synonimized to it instead of tags.
tag-request
tag uses thetag-request
tag ;) Also it seems that only 3 questions on meta are actually using this tag; so at the very least this isn't a rampant problem.