About Logoff button help???

[realistic]

I have website that is authenticated using Active directory (no form authentication-simple password protected folder that prompts for credentials).

Once the user enters his/her credentials, they have access to the password protected section. I need to have a logoff button that once the user click on it, the cookie that stores their credentials, gets modified or deleted, so if the user goes back to the link, the credentials are asked again. All this without closing the browser.