[Zope] expiring cookie

[schandra at csee.wvu.edu]

Hi,

I am having some problems with the authentication logic.My application
uses three roles--Student,Faculty and staff.I use my login page(cookie
authentication) to collect the username and
password and I use the LDAP directory  to authenticate.The authentication
works fine but when I log out and hit the back button in the browser,I am
still going to the secure page and not redirected to the login page.I
check for roles in all pages.I think my way of expiring the cookie is not
correct.
For logout, I use the following statements for cookie expiration bu tit
deosnt work. Can you see whats wrong?

<dtml-call expr="RESPONSE.expireCookie('__ac',path='/')">
<dtml-call expr="SESSION.invalidate()">

Is rewriting the cookie data is the only way?If so ,how to do that?
Thanks in advance,
Srini