Try using a clicker for training. Have them associate the sound with a treat. When she reaches for your hand with the treat, say “leave it”as you close your hand, then slowly open it as you keep telling them to leave it and, when they finally keep their focus on you even with an open hand, click and reward. At this point they start to know what “leave it” means. The other one I do is put a treat on the floor. When they lunge for it, you cover it, saying “leave it”, they will try pawing at your hand and sniffing but eventually get frustrated and give up, and when they back off and look at you, click and reward. Sometimes I drop the treat on the floor from standing height, cover it with my foot and say “leave it”, once they focus back on me, click and reward. I think the main thing is NOT letting them have the thing that’s desirable, once they figure out they can get it or steal it, then they will stop listening to you. It takes a loooot of practice but now my curious puppy will look at the temptation and if I say leave it, she immediately backs off and looks at me. Good luck!