At April 2012 there are some apparent problems with countries not being blocked.
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At April 2012 there may be some problems with countries not being blocked, possibly related to the way the geo database is updated for free users. User experience and opinion vary, and following a complaint, the original forum post about this has been deleted - http://forums.contribs.org/index.php/topic,48560.0.html
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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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Users are advised to determine the effectiveness of the database for themselves.
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Refer to this forum post for more information http://forums.contribs.org/index.php/topic,48560.0.html