It's a good move to ban the people due to cheating, pirated games, etc. Having said that, there are perfecty legal uses for a modded console. I know the widespread perception is that "modded console = piracy" but it's very similar to the perception that "Torrents = Piracy." Although a large percentage of people using torrents or modded consoles are, indeed, using pirated software, there is still a large percentage that are not.
I feel the user's xbox live should be banned, not the console itself. In addition, the second hand console issue would be a problem. After all, the person buying it most likely won't even know that it is modded.