Is your dog a puppy? Most puppies bite like that. What kind of dog do you have? Anyway-
do you get Dog Fance magazine? I do. If you do, look at page 66-67. There's an article about 4 easy steps to stop nipping and biting. I suggest that you buy your dog a SOFT muzzle, so it won't hurt him, DO NOT buy a muzzle with a basket, because it does not leave enough room for your dog to pant when he/she is soft. Or, talk to your vet, mabey they can help you solve the problem!!!
And when you are out in public, bring a chew toy, or a few dog treats, so you can distract he/she when wanting to nip or bite!!! Good Luck!!!!!!!!!!!!!!!!!