When client's book online, they have the ability to cancel their appointments (except when you have "Pay Full Amount" enabled).
However, clients cannot cancel if it is within your Minimum Cancellation Period.
You can set it up so that clients cannot cancel either 24 hours, 48 hours or 72 hours prior to their appointment.
To do this, go to Online, then into the Admin tab:
Scroll down to BOOKING SETTINGS:
From the Minimum Cancellation Period drop-down menu, select either 24 hours, 48 hours or 72 hours, and then Save.
Done! If a client tries to book within your Minimum Cancellation Period, they'll receive the following error: