Let your kids know that if they physically hurt each other there will be an immediate, consequence like the loss of a privilege or toy. Then, when you give the consequence stick by it. Doing so will let your child see that you mean what you say. This is critical in avoiding future power struggles. It will also provide the impetus your children might need to avoid the urge to fight physically.
Media violence is a big problem.
We need to counter this in our homes by providing children with an alternative reality - one where they learn concrete, practical skills. When children discover that they really can resolve conflicts peacefully, and when they watch their parents do this, the alternative reality starts to take shape. Children need to learn the skills of peacemaking in their own homes - otherwise what they see in the media becomes the only reality they know. Also, curtail TV viewing. Studies have shown that the more TV children watch, the more aggressively they tend to act.