You can't disable the back button. Nor should you be able to.
But you can stop the back button loading the page from memory/cache.
You need to set some response headers when you originally serve the page.
response.headers["Cache-Control"] = "private, no-store, max-age=0, no-cache, must-revalidate, post-check=0, pre-check=0"
response.headers["Pragma"] = "no-cache"
response.headers["Expires"] = "Fri, 01 Jan 1990 00:00:00 GMT"
Then when the user clicks back, it will request the page again from the server and you can redirect back to the login page.