Troubled kid or no, nip it in the bud. Its not about keeping him from learning jiu jitsu so he can't use it to hurt people, because there are a lot of easier ways to seriously hurt somebody than learning jiu jitsu. It is about protecting your other students and giving them a safe, comfortable environment to learn. If the gym owner is still disagreeable, I wonder how he is going to feel when this guy drives away several other students and gives his school a bad reputation in the community?
Martial arts, jiu jitsu included, aren't for everyone. Some people may respond well to being confronted about needing to change their ways, and others just need to be shown the door they have made it clear they are going to change nothing about their behavior. You can't discriminate on age, race, sex, or orientation, but you, and further more your students, do not have to put up with dangerous, immature individuals.