Our CEO has three Personal Assistants who access his Mail and Calendar Items. He does not anyone else to be able to book a meeting directly. How can he gray out his Free/Busy when someone invites him to a meeting?
I hope I understand your question. Basically what I would do is give the PA's adequate privileges to his calendar to review and edit it directly and then go to Tools | Options | Calendar Options | Free/Busy Options (on his machine) and set it to publish zero months of free/busy information to the server.
That should result in his free/busy info being unavailable to everybody, but the PA's will still be able to access and schedule things on his calendar.