In zimbra 7 web interface, go to calendar, use list view, select multiple appointments:
(1) Click [delete], a "Informational" windows will pop-up:
You have selected one or more appointments which are read only. These appointments cannot be deleted.
All those appointments can be one by one no problem.

(2) Drag&drop them to the trash bin, a 'do you want to inform originator' pop-up, make your choice, but only one appointment is moved to trash.
Anyone has the same issue?