It would be suicidal, the USA would retaliate (FWIW the USA can survive autark-ically for extended periods albeit with consequences). The world economy would come nearly to a halt, and there would, outside of the second and third world, utter economic collapse – ergo, no one is that crazy. Prices, particularly of petroleum would skyrocket, markets would collapse, central banks wouldn’t be able to clear, and ….
It would be really, really, really bad. If the USA shut off the world financial system alone, or the internet, that would pretty much end all opposition. $14T flows through the USA’s financial system every day. EVERY SINGLE DAY. That’s what, seven times Russia’s annual Economy? That’s the entire Chinese annual economy every two days?
The problem very different from business or personal finances. If you or your family or your small business stop ‘spending’ the world goes on without you. But if your whole country stops spending, monetary velocity stops and everybody starts starving really,really fast. The same is true of the world. Wanna see a great depression when there are too many humans for us to go back to villages and farms to survive? Well, it will make the great depression look like an overnight flu.
Nobody both gets to be in charge of a government, and be that stupid at the same time, because – believe it or not – there are guys like me that run the world system of production, finance, distribution, and trade, and we all understand just fine what would happen…