Firmly say no and STOP. That hurts. If he’s feeling hurt before hitting out. Acknowledge his feelings and say “You can say I don’t like it.” If someone upsetting him or her first.
Use your words. Empower them with words. Pushing hurts.

I don’t know what you are supposed to do but I would stay at the pre-school and watch them and give time out every time. I had to stay at my sons class after another boy was hitting because he was afraid of them. Maybe try to figure out why he is doing it and try to get him to understand how it makes the other kids feel.