The way I’ve seen this trick used is to put history.forward() on every page before the page where you don’t want the back button to work, then every time the user hits the back button it forwards them back to where they were. The common use is to prevent others from returning to a page (usually in a given, linear sequence) once they have progressed. This is sometimes used in the sign-in sequence for banking websites, for example.
As far as I know, there is no way to actually disable the back button. Sometimes people get around this by opening the page in a new window, which will not have a history of pages preceding it, and thus nothing to go back to. Others simply display a warning message before going back to inform a user that they may lose unsaved data, if that is the main concern.
That said, maybe this will help you: http://viralpatel.net/blogs/disable-back-button-browser-javascript/