Get some corrugated steel or better yet, some old real estate signs, and trench between the plants. Bury the sheet steel to "box in" the plants.
I boxed mine in 3' x 4' squares 18 inches deep.
And I still have to root prune. These dudes are opportunists and will find any cracks or holes they can and spread. Boxing in and root pruning are your best defense to maintain a proper hanging garden.