When our goldie "Will" was about 16 weeks we had a similar incident at puppy school where he stole another puppy's bone (each pup was given a bone at the end of the lesson) and then growled aggressively when the other pup and the puppy school teacher tried to take it away from him. He eventually let me remove it. It really upset me, however, I spent the next few weeks deliberately removing his food bowl or bone before he finished and then returning it a short time later ... the same with favourite toys. He learn't very quickly and we haven't had a problem since. We also reinforced leadership stuff, you know making sure you are first through the door and that he is fed after the human family has eaten etc Will was desexed at 6mths, he is 9mths now and gorgeous.