Had a similar situation with a player in an adult league. Once he played against all the teams, the second meeting with each team brought more complaints from the defenders. Here's what I did. During a time out, I approached the "fist" player and told him that it's one thing to protect the ball and another to extend the arm. If in my opinion he initiated the contact I would call a player control foul. He never did it again in one of my games. After the game he came up to me and thanked me for the warning and of course told me he didn't realize he was doing it. Some times just a little chat away from everyone else works better than just blowing the foul or calling nothing. I still beleive, especially in youth and rec leagues, that we are a coach on the floor and a little guidance doesn't hurt.
Have fun - -Suppref