If you don't want to allow customers to purchase and ship to a certain country you would need to make sure that you don't add that country as a shipping option. Under Settings > Shipping in the Store Admin you will be able to set up shipping parameters for all countries (or if the shipping rates are different per country you can set it up per country). In order to not enable the United States you would want to make a Parameter for North America and just not select the United States of America.

First, you will have to Add Shipping Zone.

Then you can label it North America and then select 'Add Countries'

In the list of countries scroll down to North America and unselect the United States.

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