How to remove contacts who are opening email outside of USA?

How to remove contacts who are opening email outside of USA?

In my campaign reports, I can see a certain number of emails are being opened outside of the US.  How can I generate a list of those and then add them to my do-not-mail list?  I only want to deliver to customers in the US. 

I don't have countries for all of my contacts, so I can't sort that way.

Thanks!