pour vinegar wherever you see them go. or wherever the nest is. also try spraying the nest with weed killer and an engine cleaner mixed.
vinegar should hurt their lungs when they breath it in. if it doesnt the weed killer and engine cleaner will start opening flesh wounds and infecting them. I've gotten rid of 3 nests by doing that.
I'm not sure if this is even an option by pour gasoline where they preside and cover the exits pretty good. gas fumes work wonders, especcialy on hornet nests. be carefull to make sure if its in an open area not to have any sparks or anything that could potencially start a fire.
Back in NC when birds are a problem the 20 gauge is usualy my first choice, but since your probably in an urban area its not. try picking up a 30$ co2 pistol from walmart or somthing and wait on em to fly in