Add Cookies
HttpCookie loginDP = new HttpCookie("loginVT");
loginDP.Values.Add("KullaniciId", kull.KullaniciId);
loginDP.Expires = DateTime.Now.AddYears(1);
Response.Cookies.Add(loginDP);
Expire Cookies
Response.Cookies["loginVT"].Expires = DateTime.Now.AddDays(-1);
LogOut.aspx way
Session.Abandon();
Session.Clear();
Response.Cookies["loginVT"].Expires = DateTime.Now.AddDays(-1);
Response.Redirect(ResolveUrl("~/Default.aspx"));