1. I'm looking for a function that will allow me to block a custom post being published if it's category has already been assigned to a previous custom post.
2. I want this function wrapped up in a hook/action to be used with WP Front End User Pro (https://docs.wedevs.com/docs/wp-user-frontend-pro/developer-docs/).
The 2 usable hooks are triggered after the post is created, maybe you need to find another approach,
if the user save the post with X category(with assigned post) instead of status published save with status draft