A one-time fix (I've only ever had to apply this technique once), is to apply gentle pressure onto the pet's inner lip using their own teeth as a pressure-gauge. This will not hurt them, only teach them that their teeth are capable of causing discomfort if not used cautiously.

Do NOT use too much pressure so as to cause your pet to yelp, just gently but with some pressure take your thumb and lightly press the upper lip of pet into upper canine tooth and hold for a second. It is uncomfortable but harmless.

NEVER train a dog when frustrated, angry, fearful, nor with negative expectation.

Human brain vs. Doggy brain--Remember, animals think & process things differently than us, so try to always expect animals to behave as animals, not as humans. They will "adopt" behavior preferable to us when taught with patience and praise and when they feel happy and safe.

Try to familiarize yourself with toxic people foods and dog foods to never feed your pet, and read up on training techniques as you and your pet bond. This behavioral aid is great on older dogs as well as puppies. Have fun!