Ming from Agents of Speech explains how to reduce hitting by praising moments when your child doesn't hit, even before a triggering event. For severe cases, use tangible rewards that you gradually fade toward verbal praise as the child builds self-regulation skills.
Chapters
The third thing I want you to do is to reward and praise when the hitting doesn't happen. So sometimes when a child is triggered, your first word is like, oh, good job. Very good. Oh, you didn't hit mommy. That's very good. Good boy. So you praise and reward before the behavior actually happens, and you give that attention before the hitting actually begins. Then there's a much higher chance that the kid will actually not hit. Sometimes when we're dealing with a severe case and we're talking about a child who is on the spectrum, who hits for everything, I'm thinking about all the times I got slapped. Sometimes we go as far as using treats, like teaching a little pet a new thing. We trigger them a little bit and if they don't hit, we treat them. That actually works a lot. But we're not going to use it forever. We're going to start thinking about like, how to reduce the rewards, how to use it, how to only use verbal praise, how to get rid of those verbal praise, how to teach a child to self regulate, so on and so forth. It takes a long time.