WordPress blogs running on multiple authors can restrict authors from posting to some specific category that admin configures in their profile. Restrict Author Posting is the new WordPress plugin made for single category restrict function for authors user roles. You can very easily assign restrict category for authors by editing their profile and the plugin will show a warning message to them whenever they try posting to the restricted category.
How To Use Restrict Author Posting Plugin?
Install and activate Restrict Author Posting plugin, upon activation visit your Dashboard -> Users -> All Users page and open any author for editing and select the category you like restricting for that author.