Toddler beating people

my Lo is 18 month, he have a habit of beating people since younger. Recently teacher start complaining him again. As now he transiting from infant to playgroup.Teacher day that he tend to beat other children when he is back from playgroup. I think he is excited to be back to infant , that why he do that. But I have always tell him we cannot beat people but till now he still doesn’t listen. When you say don’t beat the more he will beat. What should I do? I am worried