From the accepted answer at Should any Meta post that calls out a user by name be deleted, not just downvoted? :
There's a reason we always kill off the specific-user tag every time it appears.
However, it seems this doesn't actually happen. As of now I see two questions thus tagged, one of which is from all the way back in 2011.
Shouldn't the tag simply be blocked, so that such "killing off" is not required? The system also currently allows me to type such a tag in the list on this question, and it will appear as a tag (of course, I am not submitting the post with that tag).