How would they exactly block you from doing business? Unless you have a physical presence, if all your transcations take place online, how would it be different from doing in the US and outside?
It may all look like bits to you, but here's something to think about: I have recently bought an electronic product from a company in a different country. The payment processor though added Canadian HST tax on top of the purchase. Processor did two things: it figured out I'm in Canada so the HST tax may apply, and it knew that the product was actually sold via a reseller, which also registered in Canada, which made HST mandatory.
You see? The online purchases are not as unregulated as they may seem. There are laws about them, and the online companies do follow those laws.
Trying to sell banned service to US customer is not much easier than trying to sell banned food. One may try but the interested troll only need to send a cease-and-desist to yet another payment processor one uses to sell to US, and the service is on square one, again.
Court order will prohibit the sales. The decision will be delivered to whatever representatives the company has on US soil, as well as to customs, payment processors and whatnot.
