I'm not sure the best answer to this one.
I don't think you can disable the back button. You might be able to clear the back button history with CSJS. Not sure.
What I'd try first might be when the user clicks save - redirect them to a new page with maybe a message that their doc has been saved.... not sure .
Maybe another thing you can do on save is redirect to the current page... maybe that would solve the back button issue...