By default you can select multiple post category while you are publishing a new post. You might want to give permission to your blog editor to post in only one category per post. Categories Metabox Enhanced is the perfect Plugin to Limit One Category Per Post in WordPress. This plugin converts the category check boxes to radio button or list style and force user to publish a post in a single category. Lets see how it works.
After installing the plugin go to settings me and set the necessary options. We have select to show our category as radio button:
Here is the category property which is changed as radio button:
Now user cannot select multiple category at a time. You can also select list style rather then radio button.