add_action('template_redirect', function() {
  if (is_page('208')) {
    $referer = 'https://digital-jinja.com/sanpai/';
    if (!isset($_SERVER['HTTP_REFERER']) || $_SERVER['HTTP_REFERER'] !== $referer) {
      global $wp_query;
      $wp_query->set_404();
      status_header(404);
      nocache_headers();
    }
  }
});