First of all,
you should never try to disable the back buttons, because yo won't get the expected results. There are many variables and you can't take care of all, for example, the user can use mouse gestures to go back, or some keyboards have the back buttons, etc.
WHat you can do is to set a session variable and when he perform the action to go forward you set the variable to true, and check for that variable in your action class if he wants to go back.