If an attendee can no longer attend the booking, they can go to the online portal and cancel themselves. Alternatively, you can manually remove them.

First of all, you need to locate the booking in the clubs diary,  

Select Diary from the menu on the left of the screen.

Locate the booking in the diary and click the relevant panel to open the event settings.  

Locate the individual that you wish to remove and click on the rubbish bin symbol. 

The individual will be removed immediately and will receive an automated email advising that the booking has been cancelled.

If they had purchased a credit for that booking, this will be made available to them to use on another booking.

Only admin users can refund the payment (via Purchases)

Also see
​Adding an individual to a booking