Go after the wasps in early June, when the nest has been established but is still small. Night is the best time because all of the wasps will be in the nest. It is also better to go after them while the nighttime temperatures are below 50 degrees F, as wasps do not fly well in colder temperatures.
Wasps build their nests under the eaves of houses and in trees. In June, the nests will still be small, so they will be harder to find. Check out all of the possible nesting places and do so more than once in case you miss them the first time around.
Use a spray bottle and douse the nest with kerosene. The fumes alone will kill the wasps. Alternatively, light a small fire under the nest. The smoke will begin choking the wasps, forcing them to leave. Once the wasps are gone, knock down the nest, so they will have no place to return to. For a less extreme method, try cucumber, which repels wasps. Cut up cucumbers and leave the slices on the outside window sills, around the barbecue area, and in other problem areas.