Reduce the number of genre tags to just ONE

I have many mp3 files with multi genre tags. the tags look like this: "Pop/Disco"
I would like to delete all tags but the first tag. In this case I would like to get the following:
How can I do this with mp3 tag?

If got this multi tags because I used the Musicbrainz Picard Plugin lastfm-Plus. Unfortunately I don't know, how to tell lastFM Plus to add just one tag. The problem is discussed here:

Thank you!

Make a new action:

Action type: Replace with regular expressions
Field: GENRE
Regular expression: ^(.+?)/.+
Replace matches with: $1