If you try to be fair, moderately prompt and consistent, you will get few complaints. See, for example, the compliments that Jim Lippard's Primenet gets. You don't need universal arbitration. We're not having that much difficulty defining spam here. Not always true.
Many providers follow the common carrier model.
This is not just Internet, it's common carrier in general. Airlines are common carriers. My machine is in the path line. I can't do anything about your user. They are your customer.