How sure are you that it is an option? I followed the link you gave, went to search settings (here: http://www.google.com/preferences?hl=en) and found no option to disable it. I went to the dashboard and attempted to find an option for it, but did not see it anywhere. At this point I am under the assumption that while this may have been an option, it's been removed -- and if this is the wrong assumption to make, then all I can say is that it should not be this difficult to find it.