You'll just have to return an empty set of rates when the checkout requests rates from your service provider. In the least, you could have a white list of countries/regions and if the customer isn't coming from there return a blank set of rates immediately.
The request will have to go to your server (which is providing the rates) and you'll need to do all the necessary calculations (even if that means doing nothing).
I understand that it's a bit of a gross experience; the user gets to the 2nd page of the checkout only to find out that they aren't even eligible to purchase the product. Though, you can use this as an opportunity to see what other markets are interested based on the abandoned carts :)