How to convince Modern Warfare 2 to NOT make me host?
Solution 1:
That's pretty easy. If you're behind a NAT (you had to forward your ports) you can just undo the forwarding or disable UPnP and your NAT should go back to Strict.
If you don't, you can simply use a firewall and set it to allow Outbound connection only, which lets you connect to servers but doesn't let other clients connect to you.