Does anyone know of a inexpensive NAT router that has the ability to limit access to the Internet to a specific MAC address?

I know the Linksys routers have a MAC filtering feature, but it is the opposite of what I need. It allows you to block access to a specific MAC address. I need something that will block all, but allow an exception.

I'm dealing with some VOIP issues in my company's network, and I think the answer is to have a separate router on the network for my PBX to use. I want to make sure that other nodes are not allowed to access the Internet via this second router.