There is no setting that will prevent you from doing this if you are a delegate. Delegation was meant as a way to grant someone total control over your calendar to manage it on your behalf. If this is not what you need to be doing for your boss then have him remove you as a delegate and grant you permissions to view his calendar instead. He can do this by right-clicking his calendar on the folder list and going to the sharing permissions. He can grant you permissions there. If he wants you to manage his calendar and accept and create meetings on his behalf then you will just have to be careful never to delete meeting requests that are his.