Hi Ron,
The definition in the WLAN Service will work as the catch-all policy. Policy has precedence.
So in your example, if you want to capture all the traffic on the service except specific roles, simply set the service to 'Enable' - both directions recommended to get by-directional view of the traffic.
For any Roles you want to exclude, simply set their default action Traffic Mirror to 'Prohibited'.
If you have both Role and Service set to Enable, then there's no discrepancy and any traffic from that role on that service is N-Mirrored.
Paulo