How to prevent others from placing Invitations on my Calendar

[Attila]

Users have been sending Outlook invitations to a manger in our organization.
We do not want anybody to place invitations on his calendar except for his
assistant. We are using Exchange 2003, but even others outside the org can
schedule events on his calendar. In fact, per our test, sending an invitatio
to anybody shows up on their calendar. Is there a way to prevent this?

Thanks,
Attila