> How do you stop them? We're talking about extremely motivated people here.

Fences. Walls. Snakes and Alligators. Guns. I think there are lots of ways to keep people in one side of the border. In addition to these (to be sure that my sarcasm gets passed, not just in addition, but much more importantly) Clear incentives. In the charter city you can work and study and live officially a good life. You go past the snake pit, you are not able to do any of that but are immediately brought back to charter city when caught.