wouldn't this just be a list property and your rule will be a 'is not one of' ?
Configuration Object Types
I am looking in the above drop downs for anything related to "Backup Operators" but I cannot seem to find anything. Its almost like I have to build it in the dictionary somehow.
I have found this to be a resolution:
in the drop down "configuration object" you can create a "new configuration object" which then let me live browse the server, checking Backup Operators:
"Windows Group:Backup Operators"."User Members (Windows)" = null