We had the same problem with our boxer pup. We had a graveled back garden it was a nightmare to let her out there! [:@] We used the pet corrector spray every time she picked up a stone, when she dropped it we used the clicker and treated her. As we have 2 boxers we have been training them along side an animal behaviourist/ trainer every week since they were small pups, we have had other dogs in the past and I have to say that using these methods personally have proved to be very successful. Our youngster is now 10 months old and doesn`t pick any stones at all which is a relief . [:D] Best of luck..if you need any more help or advice pm me.