It is possible to prevent an order for someone from a specific country. To do this, you will want to use the Active Countries feature.
Basic Terms and Definitions
Active Countries - A list of available countries where customers can make an order.
To go to the list of available countries, you will need to log into the admin panel. Once logged in, visit Settings > Store Information > Active Countries.
Here, you will see a list of countries. To disable countries you would not like to take orders from, check the country on the list. Once all the countries are hidden, click More Actions > Hide Selected in the top right. Note that if you want to add these countries back in the future you can do it here.