BACK buttton is a property of the client browser and cannot be controlled from server side aspx page/code
I am not aware of anyway to disable the BACK button. But you may be able to use JAVASCRIPT and clear the history. When you clear the history, the BACK button will be automatically disabled
Another think some sites do to get this done is, open a new window from your site without the toolbars. In this case, users cannot see the back button. But smart guys can press the 'BACKSPACE' character in keyboard and go to the previous page