To kill woody plants, you will need brush killer, ordinary weed killer won't get the job done.
Make a fresh cut and paint, drizzle or somehow cover the stump with the killer diluted no more than 1/2. Go back and recoat the next day.
Or............if you have a pesky tree that needs to mysteriously die, drill 1/4" holes at or just below ground level and use a baster to squirt the full strength stuff in (don't take the baster back to the kitchen). I helped out a neighbor 30 years ago, when his neighbor proudly planted a weeping willow tree adjacent to the property line...about 15 feet from my bud's septic field. For those not in the US, the weeping willow is notorious for sending roots a half mile to a water source.