Change what a timeslot says on the booking form if it is booked.

Step One.
Navigate to the settings option, and select the 'Booking Form' wizard.

Step Two.
This setting is found on the 3rd tab 'Slots'.

Step Three.
Scroll down to 'When a timeslot is already booked or maximum slots are filled, your customers will see the following message' and enter what message you want displayed. e.g "Sold Out" or "Unavailable"

Step Four.
Click Save at the bottom of the box.