I would suggest two things.
1) Focus on moving something else. In other words, start the downswing with your lower body and keep your core rotating through the shot. When you don't use your body (hips) or if they slow up or stop during the swing, then energy has to go somewhere, so it transfers into your arms and hands too soon. If you simply leave your arms and hands alone and let them fall naturally by letting gravity takeover and be aggressive with your core, you should stop throwing it from the top.
2) When you make practice swings exaggerate a super wide takeaway and then a narrow downswing where the shaft moves in closer to your right shoulder. Most who cast are narrow going up and then because they have no place to go get wide coming down. Wide to narrow should be your mantra. And you have to really exaggerate it and just let go. If you can video tape yourself doing this in a practice swing and then compare to your regular swing and you should see a huge difference.
Also here's a drill you may try.
Description: The Double Pump Drill is a great anti-slicing drill that will help you learn to unwind your upper body while keeping the club on plane.
1. Starting with a short or mid iron, make a backswing and stop at the top of your swing.
2. Keeping your wrists and arms quiet (keeping the angles in them) pull your front shoulder out from under your chin and let your hands come down to hip high. The golf club should now be parallel to the ground and the shaft should be parallel to your toe line.
3. Pump back up and come back down to the same point a second time. On the third time once you reach waist high continue through by turning to the target and striking the ball.
4. Making these repetitive moves will assist you to bring your club down to the inside and will prevent you from casting or spinning and throwing the club over the top of your plane.