Certified GKH Gardening Expert
Cut into the roots and paint the fresh cut with undiluted Round-Up. The fresh cut will cause the plant to pull the Round-Up into its system and will kill it. You may need to repeat 2-3 times if the roots grow deep, but it will work.
Perhaps this article may be of some help: https://www.gardeningknowhow.com/ornamental/trees/tgen/how-to-kill-a-tree-killing-trees-in-your-garden.htm