How to Stop Relay on Exchange 2000

I have my exchange setup, but there are people who are relaying through the exchanger, which is choaking alot of our bandwidth.
I’v tried by unchecking the allow annonymous box, but that then denies us to recieve mail from an external network, i’v tried to add allow only the list below check box, but then that stops POP and does not let the MAC’s on the network to send and receive mail.
Is there any viable solution,so i can stop people from relaying through my Mail Server, and have a happy Exchange that will let us do POP and External Emails :-).
Thanks
Saaed