Are you sure you can’t set the permissions for the parent category so they are viewable only by the users you want to limit access to? You would put them in a group and then give that group read access but not read and write access. If they are seeing that category in the composer as an option then maybe there’s a bug, or maybe they need to do a hard refresh in the web browser after you change the category permissions.
I would also recommend hiding the new topic button in that category, it caused a lot of confusion on my site having it there but greyed out. Just add this css and change the category slug, I assumed your slug was classes.