I'd suggest you to look into some geolocation/ipaddress database such as http://www.maxmind.com/app/city to deny access to the signup form for "foreigners".
They have two editions of their database, a commercial one and a free one which is inferior to the commercial product (in terms of accuracy). They also offer code to read from the database file or a webservice (which is not free as far I remember).
At work we're using the commercial product and we're quite satisfied, despite not being US residents and thus not having the luxury of zip codes.
Slashdot Mirror
I'd suggest you to look into some geolocation/ipaddress database such as http://www.maxmind.com/app/city to deny access to the signup form for "foreigners".
They have two editions of their database, a commercial one and a free one which is inferior to the commercial product (in terms of accuracy). They also offer code to read from the database file or a webservice (which is not free as far I remember).
At work we're using the commercial product and we're quite satisfied, despite not being US residents and thus not having the luxury of zip codes.