You don't have to approve every booking you received from the clients. If you had a bad experience with the client, you can simply decline his booking request. Or you can also cancel an already confirmed booking. It takes just few seconds!
The confirmed bookings can be cancelled in the window of the event (see Calendar settings for further info). Just click on the cross button next to the appropriate booking. If there is an email assigned with the client, the system offers you the possibility to send him a polite, but clear message, about why you have cancelled the booking.