Advice on growling puppy on walks?

Hi all,
Just got back from a lovely walk with Sam and Maia. The training is going very well; the pulling has improved drastically, and they walk well on the joiner. They sit off to the side when a bike or other person is going by. BUT... I'm starting to worry a bit about Maia.
When she was only 9 weeks old and I took her to the neighbourhood park for the first time, she put her little body between me and a fenced in dog and growled her little face off. Now, she is 5 months old. On walks, if she sees a person in the distance, her body stiffens, her gait changes, she tries to walk ahead of Sam and I and put herself between us and the other person. Then she starts to growl. She sounds horribly vicious. If I talk to the other person and bring her over, she stops growling and is fine. If I try to keep walking, she turns into a tornado of spite, trying to get at the person. If I stop and have her sit, and step between her and the person (something I was told to do by a trainer), she continues to growl and tries to get around me. I try to distract her with yummy liver, which she devours and quickly begins growling again. She is small enough right now (35 lbs) that I can easily keep her in check, but I'd like to nip this in the bud before she's 60 lbs of pure muscle. Besides, she's very scary looking... this is NOT alright with me. Incidentally, Sam does nothing while she's doing all of this. He calmly sits and watches. Not tense or concerned at all.
I have no idea why she does this (well, obviously to guard Sam and I... but why does she think she should??) She is a very friendly little girl (boxer) to everyone she MEETS. (there has to be an introduction...) She's never to my knowledge been hit, or had a bad interaction with anyone. She does this to kids, adults, men, women. (not to bikes or blades... just walkers and runners).
Any advice?

I notice you posted quite a few days ago, but didn't get any suggestions...

My dog did this too when out for walks. God forbid anyone else try to take a walk at the same time as us... Tucker turned into a nutcase. He got super-over-protective, growly, snarly, and even tried to pull towards the person walking off in the distance.

I did the same thing your trainer suggested, put myself between the dog and the person in the distance. However, I also used a series of other commands to get and KEEP my dog's attention on me:

ENOUGH = stop making noise (growling, barking, whining...)
WATCH ME = make eye contact w/ me
SIT/WAIT = sit nicely, and wait for further instruction
TOUCH = touch nose to my hand
OK! LET'S GO! = continue on w/ our walk, this is the reward for following the commands

there only reason I used TOUCH is because it gave my dog something to DO at that exact moment, to redirect his attention immediately.

I found giving Tucker a string of commands helped him focus on me, took his attention off the other pedestrian, and reinforced all of the commands.

keep exposing Maia, redirecting her attention to YOU, and praise the good stuff. (sometimes easier said, than done.... good luck!)

Patience and basically what Jessi has said will get you there. My guy did that in the fall to people walking dogs after a few run-ins with less than friendly dogs. We worked all winter and now he doesn't make a sound, he sometimes pulls to go see but he will LEAVE IT and LETS GO rather nicely. I think the most important part is keeping the leash slack so the dog doesn't feel your tension.

Thanks so much! I was :sad: not to get any responses!
I will definitely try the 'touch' thing. that sounds like a great idea. She's such a nice dog that it is surprising that she acts this way. She is getting better, but yesterday she went insane at a guy... it seems worse with guys for some reason.
I've been told not to soothe her (ie. it's ok...) or pet her when she does this, but it is hard not to!! I'll be strong...

I've been told not to soothe her (ie. it's ok...) or pet her when she does this, but it is hard not to!! I'll be strong...
Stay strong, you're doing it for her own good :-)
I had the same problem with Buster, he used to lunge at men if they 'dared' walk too close to his mama!! I did the same as Jessi with the comand 'enough!' Consistency is the only thing that works. You gotta remember, Boxers are a protective breed, she's only doing what she's been bred for in the past. I find it's kinda like teaching a 'barky' breed to stop barking. You can't stop the behavior completely, but you can keep it in check. Buster is allowed one little growl, but then I say 'thank you, now ENOUGH!' and he stops right away.
Good luck...Maia is such a little cutiepie!!