Unfortunately, that is not a feature that really exists in kiwitrees (or webtrees). I guess it could, but it is quite a complex problem. Similar to the “clippings cart”, but with a delete rather than download option. I might get a chance to look at it one day, but not soon.
However, there is a good solution that I have been using for many years. It involves downloading a fresh GEDCOM file exported from your site; use a simple tool on your local computer (see below) to remove the branch you want to delete; then re-import your tree back to kiwitrees.
The tool I use for this is “gedsplit”. It’s been around for years, but amazingly still works even on Windows 10 (not sure about Apple computers though). You can get a copy from the kiwitrees downloads page (https://kiwitrees.net/services/downloads/)