When we're not civil, we're letting go of our own civility. Two wrongs don't make a right. However in practice, it's tough to figure out the appropriate coarse of action. If you treat the other person well while you're being treated badly it could be seen as acceptance. So the only solution is not to interact with that person at all, to the extent that it's possible.

And then there's practicality. Just the other day some man called me rude because apparently he said hello to me and I did not notice, let alone respond- I only noticed when he started yelling at me. Sorry, I am under no obligation to be friendly. However, I suppose that in theory it does seem rude- I mean a person gives me a friendly greeting to me and I completely ignore him. So, technically uncivil but perfectly acceptable once you take into account the environment. The fact that he started yelling at me is reason enough not to feel bad about ignoring stuff like that in the first place.

But wait- according to your social contract theory, I was uncivil to this man by not greeting him back, so he was perfectly within his rights to be uncivil to me by yelling at me?