The Latest DDWRT has Xbox Live listed on it's on in QoS, you could block it completely from there

It's just a drop down. (Entirely block it)
Alternatively you could block Xbox Live and filter it solely to the Xbox's MAC - also DD WRT.
Martinchris23,
Around me at least they're holding their value right around $100. But you are correct about just buying a spare if you're that concerned, I picked up a busted one (bad hard drive) for cheap and fixed it for Xbox Live use.
sorry to revive a dead thread, but i found the service built into ddwrt does not block xbox live at all. atleast it didnt for me. tested this on my live 360 and it would still connect even with upnp disabled.
i setup my own services (2.. 1 for 88 and 1 for
udp/tcp) and it then blocked live. upnp can be enabled as well it doesnt bypass it.