Tags are removed from the database automatically about a day after they are no longer in use. The solution is to retag the question appropriately so it does not hold on to ill-advised tags. If the community does its job, the question should eventually be deleted. That will take care of the problem, too.
And, no, adding place-holder tags like [off-topic] would not be appropriate. Those are meta tags and they are discouraged: The Death of Meta Tags.