I'd talk to the local police about it, and see if they can't set up some kind of "sting" operation -- have a plainclothes officer hang around out-of-view of the parking lot, and when the thieves make their move, nail 'em. Should prevent the problem in the future...

Either that, or hire someone to hang around in a nearby tree with an AR-15, a silencer, and some decent optics. Nothing says, "Leave my car alone" like a NATO .223 round being fired through a kneecap.