I had a bald faced hornet nest in one of my pines. I waited till it was almost dark (they're less active), put my ladder up next to the nest, then took a large clear plastic bag (you'll want to see how pissed off they really get) and put it over the nest. I duct taped the bag closed around the branch, then cut the branch off the tree. Then, using a pump sprayer, I shot 91% alcohol into the bag (which killed the hornets). No need to completely soak the nest, as the vapors will kill them.
I don't recommend this method for everyone but, in my case it worked out well.