From https://confluence.atlassian.com/doc/add-remove-and-search-for-labels-136419.html:
You can't remove, consolidate or manage labels directly. A label is created by adding it to a page for the first time, and ceases to exist once its been removed from all pages it was added to.
I have removed some labels from all the pages but they still appear in the list of labels displayed by the Labels List macro.
For example, I deleted the label quarantines from all pages, yet it still appears in the list of labels displayed by the Labels List macro.
Screenshot of output of Labels List macro:

However, if you click the quarantines label, you find there are no pages with that label.
Screenshot showing no pages with the label quarantines:

This issue has some urgency as we are trying to clean up our labels before we start working with a firm that we have hired to help us with SEO.