The 'Mosquito'.

http://news.bbc.co.uk/1/hi/politics/10442500.stm
http://www.theregister.co.uk/2010/06/25/mosquito_ban/

"The UK government has rejected calls to ban a device that uses a high-pitched noise to disperse teenage gangs."

I might start playing Drum & Bass at fuck-off volume levels to disperese shuffling anti-social footpath trudgers from my local vicinity when I'm shopping in town on a saturday.

Even if you don't give a damn about the moral/legal angle, try the Whipper-snap-o-meter test and see if you can hear the sounds in question:

http://journal.plasticmind.com/ears/mosquito-tone-or-how-to-tell-youre-a-youngun/

'Mosquitos' pump out sound between 16-18kHz and are, allegedly, generally only heard by people under the age of 25.