My DS always took DDs dummy out of her mouth, I found distraction was the best way to prevent it. When I saw he was going to do it I'd invent something for him to look at or do instead then by the time he'd done whatever it was he had forgotten he was going to steal the dummy lol. Checking the mail was his favourite distraction. I'd also do things like look out the window and act as though the car parked across the road was very exciting or see if we could see any birds.