I have no experience of what blocking someone actually does, but are you sure this user is copying items directly from your tree? To explain what I mean....
I made one of my trees private a year or more ago, excluding any of it coming up in searches too, and only the other day I got a notification in the Recent Member Connect Activity box on my home page which said "2 photos from your tree were saved to X in Y's tree". It was clear, when I clicked on the link and hovered the mouse pointer over the 2 photos in Y's media gallery for X, that at least one other person had copied the photos when my tree was still public, and that's why they're available to copy as long as those other trees are public too.
I'm not complaining, by the way, although I sometimes regret having put any media on line at all.... but I think this might be how the other user is still apparently copying things from your tree, even though you've blocked her.