Administrators always have full access to all functionalities - they cannot be restricted by permissions, and there is no config option to limit deletion of users, let alone users from a specific external user registry. The only way to disable such a functionality would be by customising Alfresco on a Java level, e.g. by implementing policies / behaviours which "veto" the pending deletion of a synchronised user with an exception. It cannot be guaranteed however that any admin UI will expect such exceptions and be able to properly handle / display them to the user.
