You can go in the menu System->Zones. There, filter the listing to display only countries (top right dropdown) and display all the entries.
Click on the checkbox in the header of the listing to select all the countries. Then, click on the unpublish button.
Then, publish the USA.
Then, go to the configuration and set your main zone to the USA instead of "Rhone" so that if you use taxes, they will be applied based on that as long as the user doesn't enter/select his address after which taxes will be applied based on the address of the user.
Finally, go to Display->Custom fields so that the only country is USA and that it's the default value. And set the default state for the state field.