Are you sure you were showing all of your files? Audio > Files is a good place to do this from.
If you've double checked, then I'm not sure what's going on. MC should remove any selection from a List type field as soon as there are no fields that are using that value. So, as soon as you erase genre "ja" from all files, it should disappear and not show up in the pull down list any more.
Maybe try restarting MC? Are you using MC standalone, or in a client server environment with several MC computers?
Brian.