Handy Free Plugin: Only One Category
This is a handy little plugin created by WP Shout blogger and Press Up Inc. developer David Hayes, which you can read about here.
If your WordPress site has a policy of only allowing a post to be put into a single category, then this plugin can make sure that you and your fellow contributors stick to this rule. By switching the category check box to a radio button selector it’s impossible to choose more than one category for a post through the post editor screen.
Whatever the reason you have decided to only allow one category per post, this plugin makes it easy to stick to that decision.