[caveat emptor]

I'm going to look forward to the thread on the best way to do that.

One of my sons and I were clearing some trees when he was about 10. Attracted the attention of some hornets who had a nest 30 feet of so up in a tree. Couldn't get close enough to continue working. Tried to get rid of them. Nothing worked - rocks, sticks, 12 Ga shotgun blasts. Just made them madder. Finally decided to cut the tree down.
Get close.
Quickly make a cut and run like hell.
They would follow us out to only 50-60 ft.
Back for another quick cut. They would have nailed us except for their self imposed limit.
Finally the tree came down. They swarmed for a few seconds at approximate 3-D location of their nest, couldn't find it, and left. Nasty buggers.

[RandallFlagg]

What I usually do is head there and mark the location, writing details from a distance (size, number of nests, etc).
At about 11PM (they’re all usually at the nest, keeping the temps regulated for the night), head on back and just spray everything around the nest first within a ten foot radius. Have to do it quickly -using circular motions with the spray. Then hit the nest itself hard. Soak the crap out of it. They’re mostly disoriented from the first surrounding spray and won’t be able to swarm at you.
Use a good, reliable gallon sized spray that has it’s own variable pattern nozzle.
Return 12 hours later and see if there’s any left. If you soaked the nest, they shouldn’t be there anymore.
Some of the little buggers WILL escape. Wear heavy clothing.

There are a lot of, “Mights,” “Shoulds,” and, “Usuallys,” that you need to remember with this. Use common sense and prepare for anything.

I’ve had to do this many times, and it’s always worked to kill them and keep them gone.