apt-get install libapache2-mod-geoip

edit /etc/apache2/mods-available/geoip.conf
<IfModule mod_geoip.c>
GeoIPEnable On
GeoIPDBFile /usr/share/GeoIP/GeoIP.dat
</IfModule>

/etc/init.d/apache2 reload

SetEnvIf GEOIP_COUNTRY_CODE US AllowedCountry

 Deny from all
 Allow from env=AllowedCountry