The Abandon method should work (MSDN):
Session.Abandon();
If you want to remove a specific item from the session use (MSDN):
Session.Remove("YourItem");
EDIT: If you just want to clear a value you can do:
Session["YourItem"] = null;
If you want to clear all keys do:
Session.Clear();
Quote:
// if forms auth is used
<blockquote class="quote"><div class="op">Quote:</div>HttpContext.Current.Session.Clear();
HttpContext.Current.Session.Abandon();
HttpContext.Current.User = null;
System.Web.Security.FormsAuthentication.SignOut();</blockquote>