Stop Backdoor

[eluser]Unknown[/eluser]
After successful login I am storing a variable in session then on homepage I am checking it , whether it exists or not?, and I am also destroying the session on log out
but when I log out then I am going to log out page but when I am clicking back button I am coming back to the home page (previous page)
and when I refreshing the page I am again getting sign out.
How to solve this error and how to destroy session on Log Out Only.

Gary