How to force developers to Fork the repositories

If an employee wanna work with a companies repository, the first thing he/she must do is to fork the given repository as he/she won’t have permission to create new branches. How can I accomplish it with GitHub?

Obs.: The employee belongs to a team that have Write permission, as they need to be able to merge other people’s PRs.

The company has repositories that belong to itself --the main repo–.
But when a developer (with write permissions) start developing something, they create a new branch on the companies repository. Over time it became a mess as they forget to delete old branches, unfinished features, test branches etc.

What I want to do is: The only person that is able to create new branches is the Admin, in this case just the company itself. All the other developers, even those with write permissions, can just merge PR, etc, but not create branches in the main repository.