Deleting an entry in Outlook Shared Calendar

I have a client who uses Exchange on a Small Business Server 2003. Someone (currently unknown) put a recurring event on the calendar which shows up every week, even though the event has long-since passed.

They have logged into every single user account in the office to try to remove it, but everyone gets the same message that the event can only be removed by the original (unknown) author, and the event is locked.

It also states that the person must be logged into an Administrator account, but this has already been done at every workstation.

How can this event be removed?

Thanks for your help.