That feature is available only for calendars booked as resources.

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Sue Mosher, Outlook MVP
Author of Microsoft Outlook 2007 Programming:
Jumpstart for Power Users and Administrators
http://www.outlookcode.com/article.aspx?id=54
> When scheduling an event directly on a shared calendar (not using the
> "resource" feature), is there a way to block users from double booking? This
> problem often occurs with reoccurring meetings.