In the Delegated Administrator Console, it is not possible to merely temporarily inactivate a user. You must remove calendar services from the user. To do this, select the user from the User List page. In the Properties for this user, delete the service package with calendar service in it. This disables the user for calendar, including setting the user's icsStatus to inactive.
If the package also contains other services, you will have to reassign those services using another package that does not contain calendar.