Now, I don't normally rush about killing things in the garden - every creature has it's place in a balanced ecosystem, and the odd wasp helping itself to any caterpillars on my plants is not unwelcome. I do, however, draw the line at paper, German or common wasps building nests in my garden - allowing it would encourage their proliferation (they pose a risk to native species), and also greatly increases the risk of myself or one of the other females of the family (all allergic, though the guys aren't) being stung.
I put a large pot of water on to boil, then added a generous amount of dish soap. He took the pot and poured it carefully but rapidly over the nest. It completely dissolved the nest instantly, killing everything. No live wasps came out. A half hour or so later I saw one late returner buzzing about looking for the nest, without success. This pic is the hole under the plant where the nest was: