Dog Park Aggression

The hubs and I took the pups to the dog park yesterday. There were a lot of dogs there, which was nice to see. It wasn't so nice that a few of them were possessive with toys and snapped at other dogs who were passing by. Two dogs in particular were showing aggression toward River, grabbing her in a not so playful way (I've seen enough dogs play to know what's aggressive and what isn't). For the most part River sat near the bench we were sitting on watching the other dogs play (normal for her). The aggressive dogs would wander over and start bothering her and she obviously was not comfortable. The owners just sat there oblivious, and at one point uttered a weak, 'Stop that whoever-the-dog-was' and I pushed the dogs away from her. After a couple of these episodes, I knew it was time to go. It amazes me how some owners feign ignorance when their dog does something wrong. It's sad to know that one day their dog will pick on another that is not as submissive as River and it won't be a very good day for them.

So what do I do the next time other peoples' dogs show aggression toward mine and do nothing about it? Ask them to leave the park? Call authorities? I'm not sure of the protocol... What would you do?