How to disable back button in the browser?

[eluser]PhilTem[/eluser]
You can't disable the button in a safe way, as @boltsabre already stated.

But, you may either use session data and set a session flag once the form is submitted. For this flag you'll check on the form page before displaying the latter. If the flag exists, the user has most likely pushed the back button.
You may additionally set the TTL of your form page to 0 seconds i.e. it is expired after propagation to the browser so each time the user hits the back button the page needs to be newly loaded and not loaded from the browser's cache.