In a comment to a plugin-recommendation tagged question @toscho advised a user of reading the tag's description when/before using it.
This is, of course, a good advise.
However, let's be honest, who reads a description of something self-explanatory? If I'd want to ask for plugin-recommendation and, hey!, there's already the plugin-recommendation tag, I'd gladly use it and post my question.
Why not mark unwanted/off-topic tags in some way, for instance, graying them out (disabled-like), paint them red, and/or bold etc., as done for the discussion and status-completed.
This is just a supplementary means, true, but I'd say this actually does catch the user's eye, and maybe then he/she will be reading the description (and thus don't use the tag or even don't ask the assumingly off-topic question).
Of course, black-listing these tag(s) would be an even better way, but that is another story...