I think BCFs idea of destroying a circle of bark and mine of a dose of salt(gly or tri) is doing the same thing, destroying the cambian (sp) layer which sprouts come from. If you're afraid of the surfactant in roundup, paint it with an equatic version, which has no surfactant(alcohol, tallow etc.) If still afraid, mix up a strong paste of table salt and water, maybe a little elmers glue. Lots of flora have been killed wih salt before chemicals.
Also better tie off the boat if using a chain saw. Not much leverage.