1. Hornet killer spray or gasoline on the exit hole, at night. Soak as good as you can, but run if any get out of the nest. Come back and re-treat night after night until they are gone.
or...
2. Pruning pole, cut 'em down at night. Make sure that it is a dark night. Dark and rainy is even better. If it is nice and windy, they don't leave the nest when you start shaking it, because they don't notice the motion as easily. Let the nest crash, but make SURE that you can leave the area without using a flashlight. They WILL fly toward the light and sting you if they can see you.
Return hours later and soak the nest in gasoline, acetone, or even wasp killer spray.
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A note on stinging insects in general: As you may recall, insects have an exoskeleton made of chitin, a bone-like substance that supports their bodies. What many folks don't know, is that this is a relatively simple chemical compound that can be dissolved, much like the paint on your car, if you hit it with the right solvent.
Acetone is a solvent for chitin. When they get sprayed with acetone (or other similar solvents), their exoskeleton and their joints are partly dissolved and quit working entirely. The insect literally cannot move it's joints, so it is done for! This is why some hornet killers claim to "freeze" them before they can sting you. Sadly, most of these solvents are also quite flammable, so the hornet killers of the recent past were not much different than spraying your house with an un-lit flamethrower.
About the best solvent for killing hornets is carbon tetracloride. This was commonly used many years as "dry cleaning fluid", and you could buy it in any grocery store. It was so flameproof, it was used as a sprayable fire extinguisher. It has since been linked to liver problems in humans, so you will not likely find any at the grocery store.
...Just another knowledge tidbit you really didn't need, but might enjoy anyway.
