By default, automated messages will go out to every client and contact on a booking. To disable these messages for a specific person, follow these steps:
Click on the booking you want to disable automated messages for.
Look in the Clients or Contacts box for the person you want to disable messages to.
Click the Actions button next to the person's name.
Select "Disable automated messages for this client" from the dropdown.
Disabling automated messages is specific to each booking, so if a client has multiple bookings, you will need to disable messages on each booking that you do not want them to receive messages for.