Change what a timeslot says on the booking form once the time has passed.

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 your defined booking cut off time has passed, your customers will see the following message displayed on the timeslot.' and enter what message you want displayed. e.g "Sold Out" or "Unavailable"

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