In Outlook 2003, you can use the various 'Arrange By' options to group your data by certain fields.  Some of these arrange by selections represent a special grouping scheme (i.e. Arrange By Size in the e-mail client).

When one of these special grouping schemes is active, the field is displayed in the Group By Box, but that field is Disabled.  That means you can't drag it and you can't click on it to change the sort order.

ReportColumn has two properties (AllowDrag and Sortable) which allow you to control most of the behavior.  Is there a way to make the column appear disabled while in the Group By Box?  You could either add a new property like (EnabledInGroupBox) or disable it automatically when both the 'AllowDrag' and 'Sortable' properties are set to False.  A new property would be preferrable.  This would simply help the developer show to the end-user that no action can be performed on the disabled field.