Clear "Logon_user"

I have a non anonymous site in IIS5
The site is for collecting any kind of data from employees.
Authentication is done with the server variable "Logon_User"

Pages:

start.asp - Authentification mask appears -->
form.asp - form for data input -->
form_ack.asp - displays the data again and stores in SQL when
acknowledged in checkboxes

After data are stored I'd like to have a button such as "Log off"
which points back to start.asp.

In the samples I tried there is always the current logged in user
present even when I do a "session.abandon" or
"Session.Contents.Remove"

But how can I "reset" the current logged in user?