If you mean you want to completely stop any kind of access for non-US users then probably something in your .htaccess file to do this. Then you'll block at the server level and won't use up as much server resources/bandwidth. If you google "block countries in htaccess" or similar. I would allow US IP addresses and block the rest (rather than the other way around).
If your server has GeoIP installed that is another good way of doing this.