S.
My 10 month old is doing this too. I grab his hand, look him directly in the eye, and say no. At first, he would laugh and try again. I've started holding his arm and looking him sternly in the eye after saying no until he drops that smile. He's getting it, but it has been a process. I also have to do this with hair pulling. He's also at home with me and an only child, so he hasn't learned it from anyone.
Hope that helps!