when click Logout button user is logedout from his account

Question

hi!
i am placing one logout button in my webpage. when user click logout button. user  can click logout button, then user logout from his account. i am write coding under Logout button like:
Session["Username"] == null;
Response.Redirect("UserLogin.aspx");
my problem is if i click back button of the page then previous page (User using page) is displayed. my aim is if i click back button of the page after logout then it display only the UserLogin page. Please give a solution.
Thanks