(Amended to correct major factual blunder – whoops – and more details added from original post of earlier today).
I was fascinated to read in the Herald this morning about the anti-teenager sounds that are being used to deter graffiti artists. High-pitched sounds that only the young can hear are being used to deter people away from places that tend to get subjected to vandalism. A quick bit of research shows that these systems have existed for a while. Whether they are ethical or not, I guess is a moot point. It makes some kind of implicit assumption that all children are criminals.