Can’t comment on your Nick Cave genre - that is very strange if your own metadata isn’t the issue.
Sadly at the moment we can’t hide Roon’s own genres that we don’t use, these have to hidden as a sub-genre. I created a new ‘trash’ genre called Other and moved it into Easy listening; now home to all my unwanted and WIP genres.
We can hide unwanted genres from our own files, using the genre mapping dialogue.
Click on the edit button for the genre, Hide, then Save:
This should work on your Nick Cave aberration. 