[kh99]

You could try this: create a new plugin using hook register_start and this code:

$banned_result = $vbulletin->db->query_first_slave(
"SELECT * FROM 
    (SELECT userid, ipaddress FROM " .TABLE_PREFIX . "user WHERE usergroupid = 8) AS banned
        LEFT JOIN " . TABLE_PREFIX . "post AS post ON (post.userid = banned.userid)
   WHERE banned.ipaddress = '" . IPADDRESS . "' OR post.ipaddress = '" . IPADDRESS . "'
   LIMIT 1");

if ($banned_result)
{
standard_error("You cannot register from this ip");
// to use a phrase here, change to standard_error(fetch_error('phrase_name'));
}

I've only done very basic testing.