Although we don't have a feature to never allow clients to cancel online, we do have a little trick with your scheduling policies to help handle this.
Here is the deal: just set your Cancellation Policy to be at least 1 day longer than your Client Scheduling Policy, so that clients can't schedule online out into the future far enough to be able to cancel online.
For example, if your Client Scheduling Policy is 1 day, setting the Cancellation Policy to 2 days will do the trick! Here's how you can update those policies:
Click on the three lines in the top, left-hand corner and choose Settings
Click on the "Policies" tile
Choose "Appointment Policies"
Set the "Client Scheduling Policy" to how far into the future you'd like clients to be able to book
Set the "Cancellation Policy" to at least 1 day longer than your Client Scheduling Policy