I'm not <username>, log me out....

Hello All, I'm trying to use php sessions to logout a user that may be sharing a computer with another user. In other words. I don't want user1 to access my site and then user2 be able to access user1's account with an auto login because the session was saved. I've seen a few sites that have a URL or button that a user can click on such as: "I'm not Joe Smith, please log me out." This is what I am trying to accomplish. Can someone point me in the right direction? I have sessions implemented using username and password but, on my login page since those vars are saved in the session the user automatically gets logged in. I also would like to have a checkbox on the login page that, if checked, doesn't cache the username and password. So if they logout they must enter a username and password, instead of getting automatically logged in. Any help will be highly appreciated. Andy