You can use an IP address database, which is not infallible but works reasonably well. Just don't forget that search engine bots (Google, Yahoo, etc.) al come from US-registered IP addresses, so you will need to whitelist their IPs otherwise your site will rapidly drop out of the indexes. :)

This could be well above me - can you point me in the right direction for more info please?

Using Google Analytics i think I have tracked down the spammer I am trying to stop - from India. Basically all I need to do is block all traffic from India - can i do this without a master's degree? :)

Or...could I deliver a specific page to this country - ie - from india they get redirected to a page that says - F*ck off!