Your participants cannot cancel their booking themselves. As an entrepreneur you can of course do that yourself.
To cancel a booking, first go to 'bookings' on the left side of the menu. In the search bar you can easily search for the specific booking by entering the name of the booker or the booking number. At the height of the correct booking, click on the 'i' symbol 'view details' on the right.
On this page you can see all the details of the booking and you can cancel the booking via the green 'cancel booking' button.