Simply go into their client profile which you can access via their booking in the calendar or by clicking on the client's profile from the client list.

Then find the section of their profile with the SMS reminder checkboxes:

Here, just click/tap on the toggle to switch off reminders for this client. The toggle will go grey.

Please go to Clients, notifications and marketing - here you can turn off all reminders, edit the time they are sent and add a bolt on to provide more information for your clients.