Hi @tomdhu,
I’ve performed some cross-testing for us today on several of my sites which have the latest versions of WordPress, WooCommerce and our free Storefront theme as the only enabled software. When doing so I’ve not been able to replicate the behaviour which you describe with customers from the USA being able to purchase products.
I configured my WooCommerce sites setting like this below.

Image Link: https://cloudup.com/cPCxlnqJwqD
On the checkout page of WooCommerce it then only shows the United Kingdom as the available country and no other country can be selected.

Image Link: https://d.pr/i/8sLGxa
If this is not working for you it could be another plugin which his installed on your site which is possible letting customers select the USA as a country in the Billing Fields section.