how do I clear http authenication?

Hi
I have this 
 string sentData = Request.ServerVariables["HTTP_AUTHORIZATION"];
What gets the authorization data they put in the default login box that gets generated byt his code
 Response.AddHeader("WWW-Authenticate", "BASIC Realm=My Realm");
                    Response.StatusCode = 401;
now I am finding once the user puts in a valid password and username it seems to stay forever. Like if I stop the website and then restart it again the username and password will be still in the  HTTP_AUTHORIZATION part. So it is not getting deleted. The only way I can get rid of it storeing the password and authenication is by clearing the http_authentication by using firefox webdevloper kit what can clear data.
 I am wondering how can I do something like that in code? Like I am not sure how to set HTTP_A ...