In your store, if you only want to sell in the United States you can remove all the other countries and regions from the dropdown list in the checkout by doing the following:

1. Run SQL script

delete from Lists where ListName = 'Region' and ParentID in (select EntryID from Lists where ListName = 'Country' and text != 'United States')
delete from Lists where ListName = 'Country' and text != 'United States'

2. Clear DNN Cache

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At your service,
Dave Smith