Is there any setting available to prevent creation of a calendar entry (or even better accepting an appointment invitation) when particular time slot is already busy?

Background of this request: Our office assistant maintains calendars for meeting/conference rooms. This means that she receives invitations when someone creates a new appointment and requests one of conference rooms. She would like to be prevented from accepting such an invitation and creating a new calendar appointment if the time slot is already reserved (i.e. it is "busy"). Is it doable?

This is Zimbra 5.