Navigate to the settings option, and select the 'Booking Form' wizard.
This setting is found on the 3rd tab 'Slots'.
Scroll down to 'Minute Cut Off' and enter the amount of minutes you want before a slot cannot be booked on the same day. e.g 30 minutes will mean that a 7pm slot will become unavailable at 6:30pm online.
Click Save at the bottom of the box.