Inside of your client's profile scroll all the way down to "Client Interface" and then turn OFF the switch labeled "Allow client to re-arrange their schedule". This can also be done for all future onboarded clients by looking for the same section on the Global Setting (top left gear icon). You will find it as 'Default Client Interface'. See picture below to know what it looks like: 
Was this article helpful?
That’s Great!
Thank you for your feedback
Sorry! We couldn't be helpful
Thank you for your feedback
Feedback sent
We appreciate your effort and will try to fix the article