There is no fool-proof way to do this. The options would include using IP addresses to either accept only certain IPs (whitelisting) or block all IPs outside the geotarget area (blacklisting) - either is a huge task with limited reliability.
The best way to control the buyers is to make the site a membership only site and vet the people before you let them become members.
I assume you've posted a message somewhere on your site (like the first form they fill out for the check out process) that tells your buyers you only accept orders from the geographic area? Do you manually check your orders?