You can create a Forms Authentication logout page by creating a logout button and calling the FormsAuthentication.SignOut() method, as shown here:
Protected Sub SignOutAction_Click(sender As Object, e As EventArgs)
When you call the SignOut() method, you remove the authentication cookie. Depending on the application, you can redirect the user to another page when the user logs out. If the user requests another restricted page, the request will be redirected to the login page. You can also redirect to the login page immediately after calling the SignOut method or you can use the Response.Redirect method.