Cut the willow tree stump down to only a few inches high with a chain saw. This will expose fresh wood that will more readily absorb the herbicide. Wear protective eyewear to prevent eye injuries from flying debris.
Drill three or four holes into the surface of the stump along the inside edge of the ring of bark. Drill the holes to a depth of 1/2 inch with a 1/4-inch drill bit.
Paint the top of the stump with stump-killing herbicide containing glyphosate or triclopyr, using a small paintbrush. Apply the herbicide immediately after cutting back the stump. Wear protective eyewear, rubber gloves, a long-sleeved shirt and pants and apply the herbicide cautiously to prevent exposure. Paint the herbicide on generously enough to fill the drilled holes. This will prevent new growth from the stump and kill any sucker trees that have grown from it.