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<?php
/***
 * Exclude widget categories
 * 
 * Online Workshop --> ID: 967
 * Online Workshop Bonus --> ID: 1081
 */
add_filter('widget_categories_args','nl_exclude_widget_categories');
function nl_exclude_widget_categories($args){
    $exclude = '967,1081';
    $args['exclude'] = $exclude;
    return $args;
}