Bottom line no other person has the right to discipline your dog unless he/she is attacking theirs and your not doing anything about it, which is obviously not the case here. We had a dog in Hunter's agility who is at the same time but had a different instructor. He was extremely hyper and every time you'd release his leash for a run he'd charge across the field toward our class. He wasn't being mean but he'd jump all over the other dogs, (he weighed maybe 25 lbs). Luckily of the 5 large dogs in our class none are aggressive, they were more surprised than anything. Unfortunately their are 2 tiny poms that weigh at most 4 lbs in our class as well. He could do some serious damage just being "friendly". We were all fine initially (accidents happen, no harm done) but the thing that really annoyed us was that the owner never corrected him, she coddled him instead. After it happened 3 times in one class the pom owners had to hold their dogs and we always made sure the larger dogs were kept between the other class and any course the pom's were running. That said none of us even considered raising a hand or disciplining her dog, we simiply don't have the right. I'd definately talk to the captains/instructors Explain what happened and discuss your concerns, they might have some suggestions. It would be a shame for you to pull out just because a couple of people are jerks. The idea of flyball or any other dog sport is that it should be fun, some people forget that in the need to win.