[Simon Lloyd]

Maybe something like this in a plugin using register_start as the hook

PHP Code:

$user_ip = strtoupper($_SERVER['REMOTE_ADDR']);
if($this->registry->options['cestr_active'] && THIS_SCRIPT == 'register') 
{  
   
$result = $vbulletin->query('SELECT COUNT(ipaddress) FROM " .TABLE_PREFIX."user WHERE ipaddress = $user_ip')
if($result !== 0){
die();
}
} 


Not tested just dashed it off