You can disable confirmation emails and SMS/text reminders for appointments in Settings > Clinic locations > Booking settings. You need the required permissions to access and edit these settings.
Note that these settings only apply to appointment confirmations and reminders, not automatic patient reminders.
To stop sending confirmations or reminders for a specific appointment:
Open the appointment in the appointment calendar.
In the appointment details dialogue, deselect the checkboxes for sending email and/or SMS/text confirmations and reminders.