disallow adding files or folders in folder root, allow doing so in sub-folders, HOW????

We have a shared folder called Department Folders, which is the folder ROOT.

It contains department folders such as Finance, Human Resources, etc.

How do I set permissions on Department Folders folder so people can not create new files or folders in this folder, they must open one of the sub-folders to create or modify any new files or folders??

Thank you, Tom