Tim and I visited many dog parks, to find one that fit us and our dogs. We found one and love it, the people are nice and not snooty, many mixed breeds as well as pure bred and if the park is busy, the bigs dongs are on one side and the little are on the other. Ours are not off lease yet, as we are training them to be off leash, it is coming along well. And I have noticed that they are so much more well behaved now that we visit the park every weekend. When they are inthey car they don't bark at dogs they see(very nice for our ears!)
The park that we go to is regulated by the government and bylaws, and it works. No poop anywhere in the park, if you are caught no picking up the poop it is a $150 fine, if your dog attacks another dog, $500 fine, if your dog is not registered or have its rabis vaccine another fine, not sure how much. But this seems to work.
Maybe try and contact you local gov and see if there is somthing that can be done, like a provincial park with fines for this type of agressive dog behaviour??
Nina