Reset auth session ..

Hey all, I have a site that I am working on that requires the client to be able to logout after using the site. I have used, as a guide, the tutorial on the zend site ( http://www.zend.com/zend/tut/authentication.php ). It shows how to get the auth session going. Short of closing the browser, does anyone know of a way to log the user out sending them to a new page but also stoping them from being able to go back and look at the site. I need to be able to end the user auth session without closing the web browser, they don't have that much access to the pc. I have thought of cookies, however I seem to tie myself in knots trying to work them into this type of auth scheme. Any help greatly appreciated. Darrin