Set whitelist to a file path and then add a file with each genre that you want to keep on a new line. You can use the top-level genres in this file to base it off (except the whitelist isn't YAML, just a genre per line): https://raw.githubusercontent.com/beetbox/beets/master/beets...
Be sure to enable canonical so it converts the specific genres into their parent genre.
Limiting to one genre I don't think will fix it - what I want is to define the allowable values for genre, and then limit to one.
It's probably better with the limit one though!