Sometimes, the ability to add multiple categories to a single post can cause problems. Or maybe you’re working with custom taxonomies and you need to make sure only one term is assigned to a post. Assuming you’re creating your own alternative category selection box, you can use this to get red of the default meta box.
add_action('admin_init','remove_cat_box');
function remove_cat_box() {
//args: $categoryslug.'div', $posttype, $location
remove_meta_box('categorydiv', 'post', 'side');
}