I think I understand what he wants:
The SME Server should be the only computer with direct access to the internet. If any computer is plugged into the LAN without properly being configured, it should NOT be able to connect. Instead, it would have to be configured to use SME Server's squid proxy [or other software] to even get out on the internet. This way, all traffic would have to go through SME Server and there would be no way to avoid, short of unplugging the server.
Well, correct me if I'm wrong I guess...PS: I still don't know how to do this I'm just clarifying this thread.
Craig