Be careful not to let the child think aggression is acceptable. Do not be aggressive yourself, but rather always maintain your self-control. Do not subtly condone violence by not setting strict enough limits on behavior. Prevent aggressive action before it arises. Then reinforce alternate, positive behavior.
Close supervision is paramount.
For example, it is far easier to prevent grabbing and to reinforce sharing than to halt aggressive behavior. Explain to the child in very clear terms that violence WILL NOT be tolerated. Explain to them that it is painful and wrong. When you see aggressive behavior, intervene immediately. Help a child to understand that there are other outlets other than harmful aggressiveness.