Once created tags persist even if they are no longer used on any page

A fellow group member of mine just reworked our complete wiki structure and tried to weed out our naturally grown tag mess, but he found out, that the tags are not removed from the tag suggestions once all mentions of the tag on any side are removed.

A good solution would be a garbage collector for no longer used tags, that either is run regularly in the background or adding a button somewhere to enable users to start tag garbage collection themselves.

Another (less desirable) possibility would be an administrational view to manage tags by hand. But I would much prefer the prior.

Thanks!