I had a beast of a wasp nest under my deck last summer. I used one of those bug-bomb thingies, where you just pop the top on the aerosol can and it sprays out constantly until it runs dry. Did that at night, and while there were still a few around the next day, the majority of them were gone. The next night I just knocked the nest off and threw it in a bag out in the back alley. By morning the remaining ones had pissed off, and then I sealed the nest with a shit ton of varathane, and stuck it back up under the deck,
I heard somewhere that wasps won't build anywhere around another nest, and I haven't had any problems since..
Who knows.