That you don't like the behavior is your most effective message. It helps any young child who has earned the disapproval of a parent to be reminded that she is loved even when you don't like the behavior. When your young child is playing with other children, keep an eye on the situation but try not to hover. What begins as playful scuffling or run and chase or sharing toys can quickly move into a battle between children, and they may need a referee. However, there are times when you can let young children work things out among themselves.
Age makes a difference, of course.
When your child is being aggressive in ways you don't like, stop the behavior and give him something else to do. You may either suggest and help start a new activity or perhaps guide him to a place where he can discharge aggressive feelings without doing harm to himself, to anyone else, to toys, or to the family pet. For example, a corner in which there is something to punch or bang or throw at can be utilized. Such an opportunity not only helps the child discharge some aggressive feelings but also helps him understand that there can be a time and place provided for such actions. When time permits, demonstrate how to handle a situation in which there is conflict between children.