master page and protected it

See more:

C#

in utility called utility i have many methods :

public static string readFromCookie(string cookieName, string key, HttpRequest req)
{
   try
   {
        return req.Cookies[cookieName][key].ToString();
   }
   catch
   {
       return null;
   }

}

public static void createCookie(string cookieName, string[] keys, string[] values, bool expierd, HttpResponse res)
{
   //    HttpResponse res =new  HttpResponse();
   HttpCookie c = new HttpCookie(cookieName);
   if (keys != null)
   {
       for (int x = 0; x < keys.Length; x++)
           c.Values.Add(keys[x], values[x]);
       if (!expierd)
           c.Expires = DateTime.Now.AddYears(5);
   }
   else
       c.Expires = DateTime.Now.AddYears(-5);


   res.Cookies.Add(c);
}

in page load of master page i write :

protected void Page_Load(object sender, EventArgs e)
{
    string user = utility.readFromCookie("login", "user", Request);
    if (user != null)
    {
        if(user !="admin")
            Response.Redirect("../default.aspx");
    }
    else
        Response.Redirect("../default.aspx");


}

in admin page load :

protected void Page_Load(object sender, EventArgs e)
{
    string user = utility.readFromCookie("login", "user", Request);
    if (user != null)
    {
        lblWillcome.Text = "willcome back   :   " + user;
        lbtnLogout.Visible = true;
    }
    else
        Response.Redirect("../default.aspx");
}
protected void lbtnLogout_Click(object sender, EventArgs e)
{
    utility.removeCookie("login", Response);
    lbtnLogout.Visible = false;
    lblWillcome.Text = "";
    Page_Load(null, null);
    Response.Redirect("../default.aspx");
}

in summary i wont to protecte my page admin so when i click the link button(lbtnlogout) i leave ihe admin page so when i click back in the browser it never back to admin page , but that code it remove cookies but it back to admin page , haw i can protect it so it never back to it >