You are able to cancel a booking from your dashboard.
Go to "List View" on your left hand sidebar:
You can view all your bookings here by selecting the date range you'd like to view.
You can also search for a booking by booking reference, event name, vehicle plate etc.
Once you've found the booking you'd like to delete select it and the option to cancel that booking will be there in a green box.
Please note that this process is not reversible. Once a booking has been cancelled you can't restore it and the time slot is opened up for new bookings.