There are two ways.Refer this link
What is the difference between Session.Abandon() and Session.Clear()
It states that:
-Removes all keys and values from the session-state collection.
-Removes all the objects stored in a Session. If you do not call the Abandon method explicitly, the server removes these objects and destroys the session when the session times out.It also raises events like Session_End.
Quote:Session.Clear can be compared to removing all books from the shelf, while Session.Abandon is more like throwing away the whole shelf.