Networking :: Redirect Marked Packets To Internal Website?

I found this really cool guide / info about creating an internet portal where users have to register (their MAC) with the server to use the internet. basically if users MAC's are in the list they get routed to the internet if not they get routed to an internal page asking them to register. It works great and works well. I was wondering if there are some smart people out there who could help me reverse the process.. eg if your mac is not in the list you can access the internet if it is, you are redirected to an internal page saying "youve been blocked"

The firewall rules are as follows.
Code: IPTABLES=/sbin/iptables
# Create internet chain
# This is used to authenticate users who have already signed up
$IPTABLES -N internet -t nat
# First send all traffic via newly created internet chain

[Code]...