Users can select multiple Categories for a post by default, that’s why WordPress use checkboxes in the Categories metabox. If you need to force the user select only one category, I have this little plugin for you.
Categories Metabox Enhanced is a new plugin released by us today. After installing it, you could replace the checkboxes with radio buttons or a select drop-down in the plugin’s Settings page. That’s how you can force the user select only one category.
The plugin is based on a useful library Taxonomy_Single_Term created by WebDevStudios, I found it when I did the research work on creating Categories Metabox Enhanced. For it’s not actually a WordPress plugin and only hosted on GitHub, I decided to integrated it to the plugin and implemented the options on the Settings.