session destroyed on logout, but back function working?

[eluser]Unknown[/eluser]
I have passed val='logout' when the user presses logout, and in logout controller function isset($_REQUEST['val']) is checked. if val is set then i destroyed the session and redirected to index page. This is working perfectly,
but if i press back button of web browser the user home page is displayed (which will be displayed only when session data is set).