Cookie not expiring...

  I'm testing an application and after clicking on the logout button
the session management cookie does not expire. More specifically, the
application is using the FormsAuthentication class signout method to
"logout" an authenticated user.

Private Sub cmdSignOut_ServerClick(ByVal sender As System.Object,
ByVal e As System.EventArgs) _
Handles cmdSignOut.ServerClick
FormsAuthentication.SignOut()
      Session.Abandon()
      Response.Redirect("login.aspx", True)
End Sub

 According to Microsoft documentation, this
FormsAuthentication.SignOut method works by returning a Set-Cookie
header to the browser that sets the cookie's value to a null string
and sets the cookie's expiration date to a date in the past.  This is
only expiring the cookie on the client/browser side. Which will allow
a "malicious" user, who had access to the cookie, to reuse it after
user logs out.

  Does anyone know how to force the cookie to expire on the server side ?

Thanks.