To send a confirmation of an appointment but prevent a reminder being sent for that appointment:

Click a space in the diary to bring up the appointment modal

Before clicking Save, select "Edit Details"

Toggle the "Only send confirmations" switch to the right:

NOTE: This will also prevent any other communications you have set up from being sent regarding this appointment.  It will not just mute reminders, it will mute all communications including communications regarding a change to the appointment or a cancellation.
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