Only one category per post

Question

How to make each post/page have one and only one category. Categories could have radio box instead of checkbox next to each.

Answer 1

Actually, I wrote a plugin for exactly that.

http://wordpress.org/extend/plugins/radio-buttons-for-taxonomies/

In the plugin settings you define which taxonomies you'd like to apply this restriction to.

Answer 2

From within the WordPress Admin, I haven't seen anyone modify category selection to force only 1 category. However, a good solution for you might be to use Gravity Forms. Using Gravity forms you can create a "front end posting solution" and in doing this you can set the category selection to use Radio Buttons instead of checkboxes. Alternatively you could also use a dropdown menu, again only allows 1 selection.

Another alternative is to modify the wp-admin area (which you will need to to in order to keep the WYSIWYG portion of the content editor as I don't think the front end solution provides this. I would suggest you remove the standard category meta box, very easily using code from this page: http://codex.wordpress.org/Function_Reference/remove_meta_box

And then create your own categories metabox using a dropdown value, which you can easily create from this page: http://codex.wordpress.org/Template_Tags/wp_dropdown_categories

Answer 3

My prefered solution would be to use Advanced Custom Field with the Taxonomy field Add-on.

This way you can add a simple html select field for category, and in the same time hide the default wordpress categories widget.

http://www.advancedcustomfields.com/add-ons/taxonomy-field/