Wally has recently begun aggressive barking towards people. He's good on walks because we have him on the pinch collar, but at the dog park, recently something seems to set him off. Its either the smell, the colors white and black, hooded sweaters/beanies, or something about the person, but he'd run up to them and start barking, and as I call him or tell him "no bark," he runs circles around the person, and recently began nipping/biting. I know its the natural herder in him, but its become more aggressive. Does anyone else experience this? Any suggestions?? I can't have a biter on my hands...