Well, one option is to make it illegal to discriminate based on nationality. This is the approach taken in the European Union, you are simply not allowed to give a different deal to someone from another EU country (with an exception for cars, that the Germans managed to sneak in). It is not enforced everywhere yet, but several good things have come out of it.

You could conceivably make similar rules on the UN or WTO level. The only ones hurt would be Hollywood and large multinationals. Which is why it will never happen.