I think that if you touch the ball & refuse to give it back, it's classed as theft. If you just leave it lying there untouched, it's okay & there's not much they can do... although I'm sure they'll just climb over & trample things more as soon as your back is turned.
I suspect that it's another pest you have to live with - we get pigeons, squirrels, rats & drunken teenagers for one weekend a year (music festival), you get urban yoofs with their balls.