Mac OS X (affects Safari and presumably other WebKit-based browsers like Chrome): run the "Keychain Access" app in /Applications/Utilities. Select "System Roots" from the upper left pane, then "Certificates" from the lower left. A long list of root CAs should appear in the middle pane, double click "CNNIC ROOT" to edit, expand the "Trust" triangle, change to "Never Trust", close, and authenticate if needed.