Imports System.Web.Configuration

Partial Class _Default
    Inherits System.Web.UI.Page
    Protected Sub LogoutButton_Click(sender As Object, e As EventArgs)
        FormsAuthentication.SignOut()
        Session.Abandon()

        'clear authentication cookie
        Dim cookie1 As HttpCookie = New HttpCookie(FormsAuthentication.FormsCookieName, "")
        cookie1.Expires = DateTime.Now.AddYears(-1)
        Response.Cookies.Add(cookie1)

        'clear session cookie (Not necessary for your current problem but i would recommend you do it anyway)
        Dim sessionStateSection As New SessionStateSection
        sessionStateSection = WebConfigurationManager.GetSection("system.web/sessionState")
        Dim cookie2 As HttpCookie = New HttpCookie(sessionStateSection.CookieName, "")
        cookie2.Expires = DateTime.Now.AddYears(-1)

        Response.Cookies.Add(cookie2)
        Response.Redirect("login/Logout.aspx")
    End Sub
End Class