I was intrigued by your question, and did a little google, which pointed me to an interesting item:
"banning by country doesn't work well for several reasons: First the domain name for the requesting IP must be available in reverse dns in order to get the country code as remote_host. That's not always true, and reverse DNS is slow. Second, you are banning an IP assigned to the ISP, not necessarily the user or a group of users. The user and ISP could be in different countries by virtue of the international nature of telecom. Even banning by IP numbers assigned to countries doesn't work all that well. There is no central "map" to tell you what address blocks go with what country - and they are assigned piecemeal and willy-nilly. There are, however, some nice subscription services to make the info available to you - big buck$, though..."
Original link is :
This sounds a little discouraging, but I thought you might be interested in this opinion