That's not how most regulations work. If you are a chemical company selling something that is legal in the US but illegal in the EU, the EU doesn't use your bank to enforce their regulations on your business in the US.

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.