Such instances are, in our experience, rare. However, if you need to cancel, you should notify your clients as soon as is practical, and cancel the session in your Helpspace calendar.

To do so, simply click the calendar event, and click the 'Cancel' button in the pop-up box that appears. A refund will be processed automatically. 

Note that it is (currently) not possible to cancel a session if the start time is in the past.