Restrict site access by country?

We've been getting alot of spam to our contact form lately and although I can restrict by IP address, the spammer keeps coming at us from different IPs. Does anyone know if I can simply block all visitors from a certain country? We only deal in the US, and all the spammer's IPs are overseas. I found The country registration restriction (http://drupal.org/project/register_country), but haven't had any luck converting that for the entire site or contact forms. Any advice?

Thanks