[Admin]

Place this in your config.php file, but change vbulletin.org to their domain (without www!):

if (strstr($HTTP_REFERER, 'vbulletin.org')) {
  sleep(900);
}

this will make the page "load" for them for 15 minutes... so unless they wait 15 minutes they will never see the page.

You can just use exit; instead of the sleep() call, but I find it more fun to just let them see in front of the screen waiting for the page to load.