I taught my ds to hold his hand up in a stop sign and say really loudly and firmly "stop, dont hit me". Then I had him practise on me half a dozen times and even practised in the car on the way to daycare. I have since witnessed him use this tactic on kids that have hurt him in the playground. While it doesn't always work or even deter at least my ds feels empowered to be able to something. It seems to work for us.