You need to log the user out, and clear the authorization cookie.
Try:
FormsAuthentication.SignOut();
Session.Abandon();
HttpCookie c = new HttpCookie(FormsAuthentication.FormsCookieName, "");
c.Expires = DateTime.Now.AddYears(-1);
Response.Cookies.Add(c);
c = new HttpCookie("ASP.NET_SessionId", "");
c.Expires = DateTime.Now.AddYears(-1);
Response.Cookies.Add(c);
FormsAuthentication.RedirectToLoginPage();
or, in VB:
FormsAuthentication.SignOut()
Session.Abandon()
Dim c As New HttpCookie(FormsAuthentication.FormsCookieName, "")
c.Expires = DateTime.Now.AddYears(-1)
Response.Cookies.Add(c)
c = New HttpCookie("ASP.NET_SessionId", "")
c.Expires = DateTime.Now.AddYears(-1)
Response.Cookies.Add(c)
FormsAuthentication.RedirectToLoginPage()