Lock a specific metadata field for all items

[VanillaXtract 3]

For my movies, I like to customize the genres. For instance, I like to add movies to an 80s genre. I have over a thousand movies, so I would like to avoid going to each movie and unchecking genres. I tried unchecking genre on the main movies folder but on a refresh of a specific movie it changed the genres. How can I lock genres for all movies?

 

Here is what I mean:

1. In metadata manager, uncheck genres in movies and save.

2. Take a movie and add a custom genre. Leave genre checked and save.

3. Refresh metadata for that movie.

4. The genres have been reset.

 

I haven't tried doing a library refresh because I don't want to risk losing all of my custom genres.