Preventing Outlook 2003 from Copying Public Calendar Items to a User's Personal Calendar

We have a public calendar that users can invite to meetings, but it has an annoying problem. When a user opens an appointment from the public calendar, Microsoft Office Outlook 2003 automatically copies the item to the user's personal Calendar folder. How can we prevent that from happening?

The reason items are copied to users' personal Calendar folders is that the calen . . .