Owners and admins can turn these reminder emails off for the whole team under Settings → Appointment Emails. While the setting is off, no contact gets a 24-hour reminder email, including for appointments that are already scheduled.
Wherever you create or edit an appointment for a contact (the contact’s Appointments tab, the Calendar, and the dialer), there is now a checkbox to send the contact a reminder email 24 hours before the appointment. It starts checked. Uncheck it and save to stop that appointment’s reminder email. A reminder goes out only when both the team setting and the appointment’s checkbox are on, and the checkbox tells you when your team has the setting turned off.
Both settings apply only to the contact’s 24-hour reminder email. Your own appointment notifications, confirmation emails, and SMS reminders keep their existing settings.