do a search on "Spoof Mac Address" or spoofing amc address.
It is doable, for anything that has a mac - although a cable modem may need specialized software not easily found.
Other than that, I'd suggest you simply buy the game, and get un-banned by steam....
Just kidding. But it could be used for that, too.
Kiddy porn, hackers, and other nefarious types have used mac spoofing software to conceal request origins. So as you start your quest, know that you'll be rubbing elbows with unsavory types.
Oddly, I had the same problem last year with Comcast. Seems the cable modem allows one and only one system access to the internet - unless I pay another $10 per month per system.... I could set up each system to access, and they could rotate through... but when the next system jumped on the net, it would block the others unless you manually rest the calls. So, I ignored their lame attempt at swindling a poor old man, bought a wireless router, put it behind the cable modem and - viola! - I run 4 systems: three at 54mbs and one at 100 mbs. The cable modem looks at the router as the authorized user, and everyones requeats run through that. Take that, you corporate rat-bastards!
So i gotta ask - what model of modem and router? maw