Do not allow user to go back after logout

If you are using forms authentication.

// Page load event of Logout.aspx
FormsAuthentication.SignOut();
Response.Redirect("Default.aspx");

//Session
//in your Global.asax file
void Session_Start(object sender, EventArgs e)
  {
    // Code that runs when a new session is started
      if (Session["UserID"] == null)
      {
          Response.Redirect("Login.aspx");
      }

no need to write any code in Global.asax file....
Use javascript to disable the back button....
<a href="http://www.htmlgoodies.com/tutorials/buttons/article.php/3478911/Disabling-the-Back-Button.htm">http://www.htmlgoodies.com/tutorials/buttons/article.php/3478911/Disabling-the-Back-Button.htm</a>[<a href="http://www.htmlgoodies.com/tutorials/buttons/article.php/3478911/Disabling-the-Back-Button.htm" target="_blank" title="New Window">^</a>]

protected override void OnPreRender(EventArgs e)
    {
        base.OnPreRender(e);
        string strDisAbleBackButton;
        strDisAbleBackButton = "<script language='javascript'>\n";
        strDisAbleBackButton += "window.history.forward(1);\n";
        strDisAbleBackButton += "\n</script>";
        ClientScript.RegisterClientScriptBlock(this.Page.GetType(), "clientScript", strDisAbleBackButton);
    }