I feel for you, my five y/o is exhibiting some of those tendencies, too. I don't know if you can just 'put a stop to it' right away. It's going to take time. Try to get a dialogue going, in a non blaming way, about what is going on in his head or his feelings when he's about to hurt someone. Maybe there's an underlying reason that you can find out and work through. It might take lots of gentle discussions to find out the root. Also, give him a solid, concrete plan of what to do the next time he feels like hitting.
Is this his first year in school? He could be reacting to a new and difficult environment. This is one reason I pulled my kids out of p.s., because my son's behavior started to seriously deteriorate after starting public school. Not to say that their behavior is perfect now, but I could tell that being in school was very stressful for him.