1. Are you sure it isn't the On Demand Email Scan add-in? I see that in my Outlook 2010, its actually its own tab, along with file, home, send/receive, folder, view, etc...
2. If you are positive it is On Delivery Email Scan, if you are disabling from the ePO console, I would run a wakeup call, then restart outlook. Also, if you are disabling from the local VSE console, restart outlook.
Just to confirm, I am running Outlook 2007, and within Outlook, the add-in is indeed called Outlook Scan. And within VirusScan console, the On-Delivery Email scanner is disabled, and this was done through ePO policy. I have performed a wakeup, and restarted outlook, and the Outlook Scan Add-In is still checked.