Most Americans would prefer not to have European court judgments against them. That said, I agree this is absurd. If I choose to do business in your country, that is one thing. But extending that to blocking my right not to do business in your jurisdiction is silly.
And if the EU started going after companies/individuals who don't have presence in the EU because they claim "we say so", well, then the rest of the world can play that game to.
Maybe the rest of the world will put sanctions on EU bureaucrats if EU bureaucrats start trying to shake down companies that have no presence in the EU.
Using the banks to cut off commerce across borders is an enforcement action for what countries agree are crimes - terrorism, money laundering, fraud, etc. It takes a lot of political willpower and negotiation to use that tool. I sincerely doubt it would be used against American websites that choose to not serve the EU.
And they got cut off from PayPal, and the credit card networks for quite a while.
If the US can use it for selling a product in a store that’s entirely legal to sell under EU law, then the EU can also use the same rules for GDPR.
For the other cases, EU uses intentional law. EU-U.S. Privacy Shield data sharing agreement for example.
In the case that the law can't be enforced directly against the violating company, EU can enforce it trough companies that provide the infrastructure for handling user data and have dealing with EU.
This includes trackers, online ad-selling companies, clouds providers, CDN provides, ISP's that have physical presence in EU and who handle user data when people visit the site. Like Google, FB, Amazon, cloufare, Akamai, Rackspace, Digital Ocean, ......
Also, if the company takes any money from the user from EU, they can get into trouble when banks with business in EU stop transferring payments.
Only if the company is handling all user data using companies with no EU presence and are not violating any US-EU privacy agreements, they should be safe.
It will become a major international incident if people try to do this.
Stuff like https://www.reddit.com/r/technology/comments/ka26b/paypal_bl... (a german company, in germany, selling cuban cigars, to a german, got as result threats from their payment processors (US companies)).
Or cases like this https://en.wikipedia.org/wiki/Society_for_Worldwide_Interban...
The US has constantly stolen money from EU citizen doing business in the EU for violating US laws.
And then there’s the Kim Dotcom case. Also similar story, US enforcing US law in New Zealand and Germany.
The US deserves some of its own medicine.