The Session_End event fires when the browser isn't connected to the server -- so there really isn't anyway to clear out the authentication ticket (since that is stored on the client).

What I'd recommend doing instead is changing the timeout duration of the forms-authentication ticket to be 15 minutes. That way you can avoid having to add a session_end event at all -- and just rely on the browser cookie timing out to force another login.

Q: Do I have a valid HttpContext in Session_End?
A: No, because this event is not associated with any request.

<system.web>
    <authentication mode="Forms">
          <forms timeout="50000000"/>
    </authentication>
</system.web>