Del old appts from cal in public folder

I am using Exchange 5.5 sp3 and outlook 2000. I have scheduling calendars in the public folders. These calendars are set up with re-occuring appointments that show available appointment times. Each appointment is then modified with the persons nameas the appointment slot is taken. How do I delete expired items from these public calendars with out deleting the re-occuring appointment itself. The size of these folders is out of hand and I need to find an automatic way to delete these old items.