Okay, when you write a post (or even after) there's a little drop down menu (triangle, arrow thing) at it's upper right. One of the options is to disallow re-sharing. You have to do it manually every time I think—I don't see a way to make that the default setting.
That's better than not having that option at all, better than email, and also annoying to have to do.
(Part 1)