I am having similar issues...me and my Exec recently upgraded to Outlook
2007. Now, I can't delete appointments I created on his calendar...instead,
I get this error message:

"Some items cannot be deleted. They were either already moved or deleted or
access was denied. This error may be the result of trying to delete more
than 4,000 messages at one time. Outlook can delete no more than 4,000
messages when it is working with a server message store.
To avoid this error, delete fewer than 4,000 messages in a single operation.
It is also possible that you do not have the appropriate permissions to
delete messages. If you need to delete content from a folder owned by someone
else, contact the owner of the folder to obtain the necessary permissions, or
have the owner delete the content for you."

I am not trying to delete 4000 messages....just one appointment. I have
checked my delegate access and it appears to be set correctly (Editor for his
Calendar and Contacts). I can delete the appointments from his computer, but
not from mine. Our corporate support technician has not yet found a
solution. Any ideas??