In billwerk it is possible to define the countries from which customers can order your product.
Go to Settings > Payment > Allowed Countries and define there in your whitelist for which countries orders should be allowed. You can either allow orders from all countries worldwide or allow orders only for specific countries. After selecting "Allow orders for specific countries", you can add desired countries from the list to your list using the plus button.
As an additional option for validation, a comparison of the payment means country can also be performed.
The payment means country is compared with the billing address of the customer and orders are only allowed if both match (currently only PayPal is supported).