Something similar happened to my dad in his youth. Dad, being the imtemperate SOB that he is, dealt with it by giving the offending party an ultimatum.
He had just dropped a 7-point with his Bubba-fied SMLE Lee-Enfield. He always carried the gun loaded to the hilt, 11 crammed into the mag and one in the chamber. He had just shot the deer and was walking to it when he was approached by two other hunters who claimed that they had just shot the same deer and that it had ran away. There was a bullet hole in its spine where the neck meets the back, so there was no way that it had ran anywhere.
My dad explained this to them, and he explained that he had just shot it through the spine at which time it fell down and stayed down.
My dad was 15 at the time, and both of these guys were older, in their early thirties. Thinking that they could get away with bullying the kid, one of them began going for his tag. This is where my dad's classic charm paid off.
My dad worked the bolt on his .303, pointed it at the pristine hind-quarters of the deer, and explained to them that they could have the deer if they wanted it. He also pointed out that he still had eleven rounds in his rifle and that he felt it was only to humane to make sure that the deer was really dead. He planned to do this by shooting it eleven more times.
The two guys left and my dad got the deer.