There will be times when your dog is going to put something into his mouth that is not appropriate, is harmful, or is just plain gross. You need a cue that will stop him in his tracks, get him to visually check in and come to you. This cue can also be used for things that you are never going to give your dog, such as the cat, your shoe, a mitten, etc.
Get out your clicker and some treats. As with any training, saying the word “yes” can work as a substitute for the “click”.
Begin by teaching your dog to resist food that you are holding in your fist. Let your dog know that the treat is there. Initially, he will nudge or lick at your hand. Do not open it. The instant the dog takes a step away from your hand, click and treat, saying, “take it.”
Move up to holding a treat in your open palm. As dog tries to take it, keep snapping your hand shut until the dog backs up. The instant that your dog backs up a step, click and treat.
Repeat this but now wait for the dog to back up and make eye contact with you. Most dogs will readily do this when you don’t give up the treat right away.
Next, put a piece of food on the floor. As your dog moves towards it, cover it with your foot and say “Leave It”. When he stops moving towards it, wait for him to make eye contact with you. The instant that you get eye contact, click and treat him from your hand. Once your dog has mastered this, place a pile of treats on the floor and walk your leashed dog near it. Tell your dog to “Leave it”. As soon as he makes eye contact with you, mark it and quickly back up luring him towards you for the treat. Never give the dog the treat from the floor. Put that treat into your treat bag or bowl. This makes it clear to the dog that they never get something that you tell you to leave. However, you are often good for a reward if they do as you ask.