Dividing post publishing (read post for explanation)

Hello,
I currently allow users to write posts on my site, which normally show up on my homepage. All fine with that.
However, I now want users to write content that will not be displayed on the homepage.
I already use Frontier Post for users to post their stuff.

So I need a plugin/method so to filter specific content (probably a category) to not display on the homepage, but I also need it to be the only category users can select when writing posts to that specific category, without messing up the settings I currently have.
So basically, I need the frontend form to limit which categories you can select, without messing up the category selection for the rest of the site.