Its a theme widget used for the sidebar
The category widget I’m seeing on your sidebar is not added by the theme. It’s one of the default widgets that comes with WordPress core. The theme only provides widget areas to place them on.
You can either add this code on your child-theme’s functions.php or use a plugin like the one below.
function exclude_widget_categories($args) {
// The numbers are the excluded category IDs
$args["exclude"] = "5,10,20";
return $args;
}
add_filter( 'widget_categories_args', 'exclude_widget_categories' );
http://wordpress.org/plugins/widgets-reloaded/