Filtering the Comment Form Allowed Tags

There’s a filter-hook that allows you to run some checking before comment is posted so you could use it too :

add_filter('preprocess_comment', 'wpse_158147_check_new_comment');
function wpse_158147_check_new_comment($commentdata){

    $commentdata['comment_content'] = preg_replace("/<tag(.*?)>(.*)<\/tag>/", "$2", $commentdata['comment_content']);// or str_replace
    return $commentdata;

}

Here “tag” would be stripped (to be replaced here with your specific tag).