Login to Backhouse
Click on Bookings > Basic Settings
You shall find the button for Block patrons from booking onlineAfter you tick this button on and hit Save, on your App, when checking a customer's profile, you will see the button of Authorize patron to book online
If you tick this option off, the customer won't be able to make online booking anymore.
When manually creating a booking, the staff on-site can easily spot the customer are being blacklisted by that little [x] icon as well.