FYI, I found the Microsoft Group Policy ADM Files, which can be downloaded from http://www.microsoft.com/downloads/details.aspx?FamilyID=92759d4b-7112-4b6c-ad4a-bbf3802a5c9b&displaylang=en and these will allow you to use the MMC to set the junk email filter properties (Local Computer Policy > User Configuration > Adminsitrative Templates > Classic Administrative Templates (ADM) > Microsoft Office Outlook 2007 > Tools | Options... > Preferences > Junk E-mail" and change/edit the Junk E-mail protection level.
However, even though I can change it here, it remains greyed out in Outlook, which is what I ultimately want to change, so that I don't have to use the management console for changes every time a user wants to change the protection level.