ASP.NET: How to end session when the user clicks log-out?

I would like to ask you on how to end a session in asp.net when the user clicks log-out because one of my professors found out that when he clicks log-out and click the back button on the browser, the session is still active and he wants that the specific session to be ended, and it that is really what to happen when you click log-out.

Thank you.