If you want to restrict users to one category per post you can change the category check boxes to radio buttons with this function:

function cb_change_cat_type(){
  echo '<script type="text/javascript">jQuery("#categorychecklist input, #categorychecklist-pop input, .cat-checklist input").each(function(){this.type="radio"});</script>';
}
add_action( 'admin_footer', 'cb_change_cat_type' );