This is precisely what I am trying to do also. I have old course sessions that I want to delete but I do not want to send cancellation messages to all the participants. (Seems to me that it is a bit silly to send cancellation messages about a session that occurred in the past!)
And of course I would prefer that this is a global setting that affects all sessions of all courses - I don't want to edit each instance before I delete it.
Perhaps if I put the site into Maintenance mode it will allow me to delete courses without sending messages? Or will it just queue up the messages and send them when I re-enable the site?