This question has the tag lightn which has no other references.
It's easy to delete the tag at the question itself, but how does one get the tag removed from the forum's list of available tags?
After the errant tag has been removed from the question (i.e. the question has been correctly re-tagged) then the (now unused) tag should be auto-purged from the list of tags - assuming that it is not in use elsewhere. This happens after a certain period of time. when the auto-purge scripts are run, which I believe happens daily at 03:00 UTC.
See this answer to How can we get rid of misspelled and unused (or “zombie”) tags?, the emphasis is mine:
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.