So far the older the plant the more ornery they seem to get. Why, just this April I cut back all the first growth on fifteen three year olds, and all that did was piss them off, making them send runners hither and yon, and sprouts many feet from their beds.
I go around each crown about a foot or so out and few inches down, trawling for runners, a few times early in the growing season. Once I get the second growth shoots winnowed down to the chosen few and up on the coir drops the plants seem to calm down with the runners. But until then it's like herding cats.
I'm not sure how far down you'd have to sink dividers to actually ensure no commingling, but I bet it's way too deep to consider doing...