To cancel multiple bookings at once open Settings > Account > Offices and click on the office where the bookings were made.
You can only do this function one office at a time, there is no option to bulk deleted bookings across the whole account
When you click on this link you will presented with a date picker.
Desks booked within the date range selected will be cancelled when you click "Yes, Cancel them"
Users will receive an email advising that it was cancelled by the Administrator.