There's no doubt that the chisel-like incisors of a beaver can inflict a nasty bite, but no...there's no way for them to bite off a hand in one bite. read more
If it can't bite fish in half, with so few bones, I really don't think it could bite a hand off. Harbour Seal (from Sea World) Sealions are much bigger but their teeth seem ill-adapted for cutting off someone's hand. read more
If this means of escape is not open to a beaver and it is cornered, then, like many wild animals, it will attempt to frighten off its attacker or defend itself or its young with a sudden lunge and bite. Beavers are herbivores that often feed on woody plants, so they have evolved powerful jaws and incisor teeth that can inflict deep puncture wounds should a person get too close to a stressed animal. read more