"Great Firewall of Belgium only allows ingress traffic from non-Belgian source addresses" wouldn't prevent IP spoofing inside or outside Belgium. Nor would doin

"Great Firewall of Belgium only allows ingress traffic from non-Belgian source addresses" wouldn't prevent IP spoofing inside or outside Belgium. Nor would doing it the other way around. If such national firewalls existed in all countries and forced all IP traffic to flow through them, it might help some.