Plugin name: Allow One Category Only
Version: 0.1
Description: Plugin replaces the categories checkbox list in the admin area, by a radio button list, so that you and users can choose only one category per post. Tested with Wordpress 3.2.2 & 4.0.1.
Plugin Download:
If this Wordpress single category plugin does not work for you, you can also try plugin named "Radio Button Categories". Thx