I've erroneously tagged some artists with tags that contain commas, see for example
https://musicbrainz.org/tag/african%20rumba,%20afrofunk,%20afrobeat,%20jazz,%20makossa,%20traditional

If I downvote this tag, the UI will rather downvote each of the 6 items instead of the full string. This means that I cannot get rid of this tag, and if I want to add the correct tags they will start-off with a negative vote.

My preferred solution is to enable complete removal of an erroneous tag, not just downvoting it.

Second best would be to allow it to be properly downvoted.

AFAIAC, the solution could be implemented via either the UI or the web service.