Hi jovceka,
Use following code to clear session and clear ASP.NET_SessionId.
C#
HttpContext.Current.Session.Clear();
HttpContext.Current.Session.Abandon();
HttpContext.Current.Response.Cookies.Add(new HttpCookie("ASP.NET_SessionId", ""));
VB.Net
HttpContext.Current.Session.Clear()
HttpContext.Current.Session.Abandon()
HttpContext.Current.Response.Cookies.Add(new HttpCookie("ASP.NET_SessionId", ""))