mobile phone jammers, great! give me 2 of them!....in fact bugger ive just though it wouldnt stop people playing crackly distorted music through them would it, darn! back to the drawing board.
or do what I do and take the phone and stamp on it (note: only if they are younger and weaker than me).
dangerous to think like that now, look at the number of dead via stabbings and shootings in london this year and last year by "youths"