It depends on your router model, really. What you want to do is alter your security settings to block traffic on ports. Visit www.portforward.com, and locate your router model there. That should help you get started. The ports you want to block are UDP 88, UDP 3074, and TCP 3074.
Alternately, although this is a huge mission, you can try setting up to connect your Xbox to the internet through a Windows machine using the built-in Internet Connection Sharing functionality. This setup usually returns a moderate or strict NAT by default.