puppy: aggressive to people

we have a 6 month old F who has ALWAYS been difficult to manage. she has quite the personality. at 3 months we had trouble with her becoming extremely aggressive, growling, showing her teeth, and biting us when we would approach her while she was chewing on a bone or something she shouldn't have (socks,etc). we saw a trainer at our vet who witnessed the behavior and taught us to press her down to the ground at her neck (on her stomach) and take the bone while saying "no!" and try it again and again until she would let us take it, praising her for the good behavior. this changed her behavior almost immediatley. now she sits on our laps and chews her bone. however, still having trouble with occasional times she has something she shouldn't and the above correction not effective anymore.

MORE IMPORTANTLY, she has growled, snared, and bitten several strangers (to her)for no apparent reason. mainly, when they were bending over to pet her. this behavior is very disheartening to us. i am worried about when we have our friends over or even worse when we have children ourselves or as visitors. our vet is saying this is typical of the terrier breed. i am not accepting this as a final answer. i want the behavior ELIMINATED! i do not want a mean dog. can you please offer any suggestions or resources? THANK YOU!