Prevent changes to Outlook calendar permissions

I have a customer running SBS2010/Office 2010.  Everyone in the office should have full access to everyone's Outlook calendars, being able to add, amend and delete entries, so the default permission level is Publishing Editor.  One user's permissions properties keep changing, meaning others can't add or amend.  Sometimes they change to "custom", sometimes "editor".  I suspect that it's the user amending his colleagues permissions to his calendar, as he tends to "dabble".  

Is it possible to lock it down so that he can't change them?  Or is there any other way that the permissions could change without him doing it deliberately?

Thanks, Ian
blueskycomputingAsked:
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EEhotlineCommented:
I don't think it's possible. You cannot stop users from doing anything in their mailbox. It is their mailbox and they have full control.
dsnegi_25decCommented:
I believe this is not possible , If somehow you are able to do that through policy I am sure you will start getting calls from user free busy information is not updating , meeting request are not updating ,etc

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