There is a new tag coupla which is a clear typo of copula. I corrected it in the one post it is used in, but how do I delete the tag itself which is here: https://stats.stackexchange.com/edit-tag-wiki/3788?
In my opinion, the best thing to do in your case is make the misspelled tag a synonym of the correct tag and merge them. That way, the correct tag will be on the thread, but that misspelling will be automatically mapped to the correct tag in the future. After all, if one person uses that misspelling, sooner or later someone else will, too. This should fix the problem forever. (Users without moderator powers should flag such threads and request this procedure when they come across the same situation.)
To answer your specific question, the SE system automatically culls tags after a while. If there are no threads associated with a tag, it will be deleted after 24 hours—you don't have to do anything. If a tag is only on one thread, it will be deleted after 6 months.