To avoid confrontation, I'd just wait until the JRT leaves before going in if you happen to see him again. There's no point in risking your dog's safety (and good temperament) because of someone else's out of control pest. That's just me though. Maybe your DP is big enough that it isn't usually an issue.
It is big enough that it is not an issue. I have never seen a dog attacking anohter dog on the small side as many times as we have been there. If I do see the dog in their I will not wait until the dog leaves. Instead I will see how the dog is acting towards other dogs from the outside of the fence if their is other dogs in the park as well as this dog. If I see that there could be a problem I will call the owners over and tell them to leave with their dog. If they do not I will simply call the rangers and have them removed.
If their are no other dogs when they are there then I will observe their dog with my dog being outside the fence. If their dog try's to attack my dog through the fence I will then ask them to take their dog out as it is not suited for a dog park at the present time. If they refuse I will then call the rangers and have them removed.
The rangers are just a phone call away and they will allow the person to return another time but if they get any more complaints after the first time then the dog and dog's owners are banned from the park.
By doing it that way I will not be risking an injury to my dog or be reversing her temperament from what it is now. I worked to hard to train her and had her certified as a therapy dog as well as her passing her CGC to have some owner who doesn't know how to control his dog or train their dog properly.
There are many JRT's I have seen at this park and Lola has played with them. This is the first one I seen like this and quite frankly I blame it on the owners.