I've used Feliway with Ripley when he began showing bad behavior after adding new kitties to the family. It helped to calm him. It also worked well after we remodeled the house and he was stressed. You can buy it at pet stores. I've found better deals for it online - it's not cheap. It comes in a spray or you can get a 30 day diffuser that plugs in your wall outlet.

There is also medication like Prozac to help calm aggressive kitties but that would be a last resort for me.