> If you use an address under someone elses domain, they can.

I suppose that's true. Google can ban a gmail account that is inappropriate on a Google Groups list, or a university can ban a student's account that represents the university inappropriately. Those are narrow cases, though. It's not in the same ball park as a mod deciding to ban someone. An instance for just the account seems the best option, but I don't know the devs' intentions.