I think that it's related to Bug #47834. You need to use the kiosk mode to not let users shut down the server. IIRC it doesn't work fine in dapper, but is fixed in Edgy. Could you test?
** Also affects: xfce4-session (Ubuntu)
Importance: Untriaged
Status: Unconfirmed
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Shutdown permissions
https://launchpad.net/bugs/59397
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