That's not a problem. profits made within a country can be taxed within a country. Stopping the export of capital is a legal/legislative issue, not a logistical one.

The rich have already left. The mythical class we're "punishing" arent tied to nations/peoples any more. Their assets are however. Their companies operate within nations. They wealth is spent in deals/properties/etc. within countries.

If a Koch brother lands on US soil he can be arrested and all of his assets seized. If he does a deal with a US company, all of the transactions can be subject to US law.

It's a matter of political will.