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For the router-imparied:
This is not a difficult process at all. Every country has a specific set of finite connections that lead into and out of the country. All traffic in and out has to go over those connections, be they landline or sat. All a router has to do is filter any traffic that passes over those points. Also, large blocks of Class A, B and C addresses are KNOWN non-US ISP addresses. Blokcing whole groups of classful addresses can also banish a vast majority of foreign browsers. There are other ways this can be done, but those would be the easiest to implement and still have sweeping effects.
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