960+Grid+–+turn+off+grid

See: http://drupal.org/node/1032486

Essentially you need to navigate to sites/all/themes/ and edit the .info file in the root folder of the 960 theme.

Just put a ; in front of the line mentioned in the first response of the above URL and then navigate to site configuration >> Performance and click on 'Clear all Caches' (this rebuilds the theme registry in the browser and makes the annoying debug grid go away).

Obviously these instructions can be reversed if you want it back.