For that scenario, you will need to use a different approach to limit shipping destinations.
In your E-junkie Dashboard under Manage Shipping, Tax, Discounts > Shipping Rules, edit each rule you have there to set Country:United States for it. Make sure you don't have any other rules there applied to Country:All or any other country.
Then under Manage Shipping, Tax, Discounts > Shipping Preferences, un-tick the box to Limit Shipping Destinations, then click Apply to save changes.
The effect of all this is that all countries will be selectable in the country menu in your cart, but buyers who select anything other than United States there will get an -error- for shipping calculation if they have any item(s) in their cart that Require buyer's shipping address, leaving them unable to proceed to checkout.
You may want to add a note explaining that in your cart. You can do that in your Dashboard under Manage Cart & Shop > Cart Preferences, where you'd scroll down to the User Text field and enter the note you want in there, then click Save Changes.
