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.
Set your Cancellation Policy to be at least 1 day longer than your Client Scheduling Policy, so clients can't schedule online 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:
Go to Settings and click on Policies
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
Save!