Turn off read receipts

I can see dozens of read receipts going out from our account which we do not want to send.

1. My Users and myself are all using Outlook for Mac which does not have an options panel to turn read receipts off.
2. However they have all been into the settings in OWA and turned off read receipts.

Despite that they are still being sent. How can we disable? We are on Small Business Edition.

Ewan