Comment Use some geolocation database (Score 1) 297
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.
