This can work well if there is a reasonable distance from the wasp nest to where you are having lunch. Squish one, and leave it. When the next one comes to investigate, squish it too. A few years back we managed to rid the world of over 40 wasps, thus making a real dent in their population

Admittedly, one of the guys was doing it by grabbing a wasp in his hand and squashing it before it could sting him. Worked quite well for a while - now he hasn't got the nerve to do it any more.