The Block Country IP is an Opencart extension that can be used to block IP address from certain countries. For example if you want to block access of the site from any IP in United States you can use this extension.
It only blocks access to website and not the admin interface so that if you accidentally block your own country you can reset the country list. This extension uses free DB from MaxMind. As per the MaxMind this DB is 99.5% accurate..