What Is the Right Thing to Do In This Situation?

Our dog walking service is called K9 Boot Camp. We take dogs on trips to the off-leash park for an hour each day. There are several off-leash parks in our area so we rotate through them, but mainly we visit the giant off-leash area at Marymoor Park. It is fantastic with the fields, the slough, the trails, etc... It has everything a dog could want (including lots of other dogs)! We visit the park every weekday morning between 8:30am and 9:00am, depending on how many dogs we are picking up on the way, and we stay at the park for a full hour. We see lots of the same people and dogs each day, which usually is a good thing.
Unfortunately, seeing the same people each day is not so fun when one of the owners has an aggressive dog. Joe and I wondered to each other about the behavior displayed by a Dobie/ Rottweiler mix that started showing up at the park around the time we did and would often run loose with our dogs. We love strong breeds and certainly do not discriminate based on reputation or stereotype (we have two Dobies and love them to pieces), but this dog was showing definite warning signs. Maybe it was never socialized as a young dog, or maybe the off-leash environment is overstimulating for this particular dog. Regardless, the raised hackles, growling, lunging, bearing of teeth, and numerous other indicators should have tipped off the owner that the dog was not enjoying itself. Initially we tried to avoid the dog, but the owner appeared to be seeking us out so her dog could burn off energy by chasing our pack. I decided to be more proactive and approach her to discuss her dog's behavior and hopefully conclude the harassment of our dog pack. She was not at all interested in speaking with me, refused to recall her dog to discourage the behavior, and walked away. Our next idea was to avoid our regular areas in the park, or hide from her really. We were afraid the dog was going to reach a point where the growling and snarling turned to biting. Since we walk several strong breeds ourselves we could only imagine what might happen to our pack, or this antagonist for that matter, if a fight should break out. No such luck!!
One morning recently, we were "hiding" in the tall grass of one of the field off to the side in the park playing with our dog pack and having a great time. Then here comes the Dobie mix trotting over without her owner anywhere in sight, typical! She approached our dogs with her hackles raised and tail pointed in the air, already not good signs. She went after our most submissive member who is such a sweet boy and began chasing and nipping at him. Even after he flipped on his back, she tried biting his neck. That was enough for me, I was not about to allow this irresponsible behavior to continue any longer. The owner was in sight and we yelled to get her attention and for her to get her dog under control. Again, we got no response. She refused to acknowledge that her dog was now attacking ours! We broke it up and as we were walking away with our group I heard her muttering "Why get so worked up, she just likes to play." I almost lost it, that is NOT playing, that is fighting and it is so dangerous and not what the off-leash park environment is for!
Sadly, we have had to take a break from visiting Marymoor because we simply cannot risk putting the dogs in this situation. I know several of the other regular dog walkers have been disturbed by the behavior of this particular dog as well. I have petitioned to the park service for some sort of resolution, since we cannot reach a constructive resolution with the dog owner herself. But I am left wondering what really is the right thing to do in this situation, what if we didn't have other off-leash areas we could visit and were forced to continue meeting this aggressive dog each morning?