On the checkout page, delete the country and shipping_country select lists. The add two hidden form elements using the same name and set the value to US:
<input name="country" type="hidden" value="US" />
<input name="shipping_country" type="hidden" value="US" />
You'll want to do the same on the users_registrsation_full.php page, delete the UserCountry select list and rep[lace it with a hidden form element of the same name with the value set to US:
<input name="UserCountry" type="hidden" value="US" />