Do not allow to search certain words

Try this in your functions.php, and change news for whatever you want to be blocked in your site.

add_action('wp', 'check_search');
function check_search() {

    global $wp_query;

    if (!$s = get_search_query())
        return false;

    if (preg_match('/news/', $s)) {
        $wp_query->set_404();
        status_header(404);
        get_template_part(404);
        exit();
    }

}

Hope it helps.