During the re-tagging of questions, tags sometimes become orphaned from existing questions.
Are these zombie tags ever removed from the tags list?
What if a tag is misspelled and needs to be removed? How do we get rid of it?
You should edit out the tag from all the questions that use it. Note that you should probably seek consensus on the per-site meta before doing so, if the tag was somehow added to a significant number of questions without anyone noticing the misspelling.
Tags not associated with any question are automatically destroyed at 03:00 UTC every day.
Tags which are misspelled should have the questions under them retagged to the appropriate tag, which will result in the misspelled tag's destruction due to having no uses.