To turn off either SMS booking confirmation/reminders or both, go to Business Setup -> Your Clinic -> Message Settings

Within Message Settings, you have the option select Booking SMS or Reminder SMS from the Message Trigger dropdown (Or select one, then repeat the steps for the other if you are turning off both) 

Within each Message Trigger, you have a Service Category drop down menu. Your services are grouped together by these service categories such as Initial Appointment and Follow Up Appointment.

Each service category is treated as it's own individual message. So, if you want to turn off SMS for ALL services, repeat the below steps for each service category.

After selecting a service category, remove the text in the body completely. After making your changes, click SAVE. This will no longer send an SMS.