Each appointment type, when you create it, has the option to send a reminder(s) at a chosen time interval before the appointment. This option appears in the "Reminders and follow-up" tab of the Appointment Type wizard (the specific appointment type in edit mode). If you do not want to send the reminder, you can remove the reminder by choosing 'remove' from the drop down gear box to the right of the Reminder and clicking save.
As a reminder, this will affect everyone who has an appointment of this type scheduled, meaning that anyone who has scheduled this type of appointment on your calendar will no longer receive the reminder. If you have only 1 client that you want to remove reminders from then it would be easier to create a specific appointment type for that client and remove the reminder from that. This way no one else is affected. Or, if you schedule the appointments for that particular client, you can leave the reminder option unchecked in the scheduling pop-up window.