My baby like to hit people what should parents do?

How old is the baby? If he/she is younger than 2 years, there is not much reasoning that you can do. In that case, get down to the baby's eye level, remove the baby away from the person, and give a light tap on his/her hand and say a firm 'no'. Keep on doing that to reinforce the behaviour. If your child is a bit older, reasoning always helps. Try to find out if he is experiencing any kind of trauma/bullying himself.