Yep, Ortho Brush Killer, probably same as TSC painted on newly cut stub will kill them. It is Triclopyr complex salt. Glyphosate will do the same, as will 2-4-D on most small trees. The Triclopyr will probably do best, since willow it tough to kill. You will still have to spray or paint leaves as the roots will keep producing shoots. You will eventually get ahead of it.