Eurotech Security offers the revolutionary and as reviewed on BBC Radio 2 the Mosquito Ultra Sonic Teenage Crowd Dispersal System:

An electronic device known as the Mosquito is now available as tested recently by the police in Surrey.
The small Mosquito box is located externally to a shop and emits an ultra high-pitched sound which is said to be only audible to young people.
The Mosquito can be used over a range of 20-25m. The piercing sound is so irritating and intolerable that young people move away within minutes yet adults will not be aware of it!
The Mosquito device is revolutionary and is designed for use in the following environments:
Outside Shop Entrances
School Grounds
Car Parks etc
Click
Here for BBC Review - Mosquito Box
Eurotech can provide a variety of wireless security systems
and software solutions including installation and technical
support.
Click
Here for product data sheet 1- Eurotech Mosquito Product Information
Click
Here for product data sheet 2 - Eurotech Mosquito Product Information