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I find it best to sneak up on them around dusk or dawn, they're less active then or can't see well, whatever the case may be. Also knock the nest down and destroy it once you've killed them it will stop the larve from hatching.
Soapy water is good. Stops them from flying and then you can smash them. Only drawback is that other wasps will respond to the death of one of their "family" members.
I have a nest in my truck and I'm sure they get confused when I drive it. I got stung on the arm the other day on the front porch. Son-of-a-guns had built a nest on the underside of the swing and I guess one got dizzy from me swinging. I drenched that nest in hornet spray and killed them all.
Oh yeah, them things will sting at night too! So will yellow jackets!