Recently, two new inventions have been developed. One, the mosquito-buzzer produces a 5 or 6 KHz buzz, which is (or should be) only audible for people under 25 years old. A perfect invention to chase street-kids or youth away from public spaces. The recent phenomena of removing irritating retired/elderly people from the street, hasn't been tackled yet. Watch this video for a proper demonstration.
Another natural invention is quite astonishing too; there have been some successful tests with spreading citrus smell in detention-centers to make 'prisoners' feel more happy and cleaner. The Dutch producer gives the following answer to the question:
"Are prisoners voluntarily exposed to your smelly gas?"
"Well, well, well, smelly gas, that's a bit too much. Daily, 4 liters of citrus smell are spread out in a detention center in Rotterdam, that's a very low dose. And the effects are astonishing. The prisoners sing and whistle all day long"
See articles in De Volkskrant and in pzc.