Yes unfortunately a clip round the ear may end in tears for you, not them.
Richard may have the best idea.
I've had them hovering myself, you know the score one tries to take attention one side while the other sneeks in. On that occasion I started a polite discussion about how I'd heard some kids had tried to steal a mates tackle and what we were going to do if we ever caught them, not directed at the two I was talking to of course as they did'nt seem stupid enough to try it, of course. But them backing off like you're on day release from the local loony bin is much preferable to losing tackle paid for by your hard earned cash.
I know, I Know, scaring teenagers is not big or clever but our hands are tied and unfortunately our society would make 'poor little soldiers' of the little thieving B?*3|\ds if we sent them for a quick swim.
I suppose how you can react is limited by how you look, but with the stone throwing bit from a bridge, "How long do you think it would take for me to get up there" Bellowed at full volume like some red faced sgt. major seemed to work.
Above all be careful, be legal.