At April 2012 there are some apparent problems with countries not being blocked.
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This would appear to be an issue with the way MaxMind is adding to or updating the geo database for free users, and a "push" to get users to subscribe to an "at cost" database update service which will provide more accurate country information.
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Refer to this forum post for more information http://forums.contribs.org/index.php/topic,48560.0.html