This plugin forces your writers to only use one category for each post by enabling the category meta box to use radio buttons instead of check boxes.
Ever wanted to restrict category usage to one category per post? I have. and so have others. Here is an easy to use plugin which will replace the default category checklist to one that uses radio buttons. This change will force all writers on your blog to use only one category per post.
It is quite possible that, if you are installing this plugin on an already existing site, you may have posts that contain more than one category. Since radio buttons allow only one option to be selected at a time, these posts will look like only one category has been assigned while in actuality, there are many. I have created the 1 Post/Category utility which can be accessed via the Tools section of the Administration Menu. Please use this tool to ensure that your posts are in the correct category before allowing other authors to post.
Please note that this plugin only changes the meta box under "Edit". The "Quick Edit" menu will still use check boxes. I plan on updating this in a future release.