How do you propose blocking a port between the cable network and your modem? You'd have to build your own custom firmware that doesn't acknowledge upstream firmware update requests.
AFAIK, they can/will kick you off the network if your modem is running unverified firmware. I think this is a regulatory requirement, but don't take my word for it. They don't want anyone to have free access to the network, you could do things like spoof your MAC address to get free service. I'm sure you could also do something much more malicious like crash parts of the network.
