How do you get an Daschund to quit biting?

We have just recently rescued an Doxie who is 4 years old. He came from a home that was provoking him to attack and bite people not in play either. we have a muzzel for him and have been working with him with out it and reinforcing that fact that no growling or biting by tapping him on the nose when he does it and prasing him when he dont. Any one have any other ideas for me? Help!!