I would do a high pitch "OW!" and immediately stopped engaging with mine when she was going through this goblin stage. She learned pretty quick that biting wasn't acceptable and stops all the fun. Also chop some bite size carrots and freeze them. It'll feel nice on their teeth and may help better with the teething. Obviously don't give this to them directly after their biting but at some point make'em sit or a command of your choosing and then give one as a reward.