Book a guest in yourself, send them to a different time or practitioner, or free the seat.
Staff can do more from a slot's "Bookings" panel: "+ Add booking" books a guest in for them (they sign the waiver at their session), "Move" sends an existing booking to any other slot in the event — a different time, a different practitioner, even a different length of service — and "Cancel" frees the seat right away.
Use Move rather than Cancel whenever the guest is still coming: cancelling and re-adding retypes their details and throws away their signed waiver.