seo – Spam, captcha – limit Contact Us page to country where you’re doing business

I do business only in the USA, however my content is relevant in just about any region of the world. I routinely get spam via my Contact Us page, almost all of which comes from outside of the US. While I do have Captcha enabled, it obviously isn’t perfect, especially when spammers hire cheap labor to manually do their work.

So, this made me wonder, what if I limited my Contact Us page to just users coming from the USA? The rest of my site would still be available.

I’m guessing I could do this with .htaccess, but would this hurt my SEO?