There is NO violence of any kind allowed in my house, and that's pretty much what my kids have heard their entire lives. We don't hit so they don't hit. When my 2 were little and testing the boundaries I do seem to remember they taking swipes once or twice. What I did was stop everything, get down on my knees so I was eye to eye, hold them gently but firmly by the shoulders and tell them with out anger, "We do not do that here. Hitting hurts so unless you want me to hit you you should not hit anyone else. I will not allow anyone to hurt one of my children and that includes you." Then they got a time out. I thought of it like this, it is easier to correct behavior in a 35 pound child than a 120 pound young man or 200 pound teenager so I always made time for it, I never let it slide... not once ever.
I think my approach works because with 2 almost grown kids DD11 and DS12, I think I am the only one of my friends whose kids do not ever beat the crud out of each other. Seriously, they do not hit or get into screaming matches. They get mad, they bicker, they argue and sometimes slam a door but there are no steel cage matches in this house. I am forever stunned when I watch older kids beating their younger siblings as the parents stand around as if they have no control over the situation...uhh, yeah ya do. Sheesh, you are the parent for heaven's sake, DO SOMETHING! Saying it's wrong without intervening is not doing something.