15 month old hitting

[HensMum]

DS has started hitting and throwing toys and I'm not sure how to stop him.
Saying no doesn't seem to have any effect at all - sometimes he just laughs at me. I've tried sitting/lying him on the floor and walking away but I don't know how long I should leave him for and I never feel like he's got the message. Often he hits after playing has got a bit boisterous so it feels like one minute he's having a nice play with Mummy and the next minute I'm walking off.
I'm trying to reduce the amount of times I say no, and distract him when he's doing something that I'd rather he didn't, like pulling books off shelves, but I feel like we have to take a firm line with hitting and throwing.
Help! I really don't know what I'm doing!

I have a 15mo DS who sounds very similar. He when through the hitting mummy phase too- usually after boisterous play or when tired/ hungry. I found the lying him on the floor and walking off strategy really effective but it took about a month of me being completely consistent. I just left him for 30 seconds max, then came back and said gentle boy, gentle with mummy, say sorry etc and had a kiss. He didn't always look like he cared much but I think that was because of his general level of frustration. It doesn't matter if one minute it's play the next you're walking off because they live so much in the moment their whole life is pretty like that anyway!

After a while I could see it coming a bit and remind him by saying gentle which usually stopped him just in time. Now when he's feeling that urge to lash he hits himself. But at least then I can just give him big cuddles/ kisses and make him feel better. HTH