If you are running a blog of wordpress, in which one or more than one user work, and you want the reader to restrict him, to post in a specific category, then you can perform this function by using a plugin, RESTRICT AUTHOR POSTING.
The way of activating and using this plugin is given below:
1. For downloading this plugin, click on this link given above.
2. After getting log on on wordpress, upload the plugin going on to plugin->add new, after that activate this plugin.
3. In this way, after going to the User, you can restrict any user to post in a specific category.