Locate the class from Classes > Schedule
Scroll to the customer list > click the dropdown to the right of the customer > then click Cancel booking.
Choose whether to credit the customer / whether to notify them / mark it as a late cancellation.
To decide what occurs when a late cancellation occurs, go to Settings > Cancellation Policy > scroll to late cancellation > add rules for auto charges for late cancellations.
Un-cancel someone back into class
From the class details page, click the Canceled tab, locate the customer, click the dropdown to the right of their name and click Un-cancel.