Short of opening a new window without a toolbar, you can't. Even then, people can use Backspace or Alt+left. Instead of trying to disable the features of a browser, you should strive to build your application so that disabling those features is not necessary. For example, if your application inserts a row into a table, and you're afraid that if the user clicks back it will insert another one, there are several things you can do (see
Article #2509).
Take a look at WHY you think you need to prevent the use of the back button, and try to solve that problem, instead of ripping functionality out from under a user's feet.
Web Men Talking recently had an
article describing the various ways to deal with this behavior.