As long as the ability to transfer money or buy stocks/invest overseas/exchange currency or basically any form of international banking exists, capital flight is possible. The government can prevent it, but they'd also have to have the prescience to know the difference between a "legitimate" use of capital overseas for business or personal use and the wholesale transfer of assets.

the U.S. can basically "financially sanction" individuals or nations. imagine it rapidly tranissioned to a multi-party socialist republic overnight (magic yay) it could leverage this same power to prevent "capital" from leaving. of course the world would try to sanction the U.S. in response but it would collapse the global economy completely. it would be a fun scenario that's for sure.

Of course the government can do that. They have the guns.

Capital outflow is very real and, for smaller countries, very dangerous. Sudden capital outflow means that your country is having some sort of crisis. Good luck getting loans if that happens.

if central banks integrate blockchain tech into their respective fiat currencies then money could quite literally be just moved out of country thru the blockchain. it’s likely they could eventually find the recipient address and demand reparations thru military intervention or simply issue them corresponding debt in accordance with the lost capital.