Fluridone would be my plan. Just Sonar or Whitecap the hell out of it. You need something systemic for sure, as that stuff roots, and mowing can actually spread it. Stuff is so delicate it is meant to break and reproduce. Some say Grass Carp won't touch the stuff. In fact, no known biological controls for it. I've never that, but some studies say so. Nuke it.
