implementing GEO-IP Blocking on Firewall

implementing GEO-IP Blocking like on comodo Firewall can be fast way to block certain ips based on any country, state or similar.
or can bee very helpful way to block some ips “from to yyy.yyy.yyy.yyy”

I am agree with this proposal but it is very difficult to implement this. How do we know if an ip is false and that we not the good ones block and how to ideal with ip and dns spoofing?