Deactivate Account and .set Identity.IsAuthenticated to false?

I need to know how to .set Identity.IsAuthenticated to false when i deactivate account?

I have a stored procedure that deactivates an account
the  is that i have on the page load
the following
  Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load         If Not Me.Page.User.Identity.IsAuthenticated Then             FormsAuthentication.RedirectToLoginPage()         End If     End Sub


on the deactivation.aspx below code
   Public Sub DeactivateUser()         Dim constr As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString         Using con As New SqlConnect ...