forcing a page to expire

How do you force a page to expire on click? I have a secure section that requires authenication and has a logout button. When the user clicks on this button, I want them to be sent to a independent page (that will be taken care of later) and once they have reached this independent page, if they hit the back button, I want it to say "Page has expired, Please login again." How would I do this? Thanks in advance. ______________________________________ s a n o g r a p h i c s d o t c o m graphics with an ultimately creative touch