This is always a tough one. One drill I've had some success with for this is races. I explain why it's bad to do this and then illustrate it by having teammates race across the court. One holding their hands together one not. Sometimes, once it is illustrated so graphically they start to get it. If some teammates are just to fast either way then time them against themselves while running both ways.

Jessica,
A little one on one with the coach tossing the ball at gradually increasing distances, alternating left and right. and constantly correcting the habit until it changes. Shouldn't take long. Make it a homework assignment for the parent and player. Stay positive!

I coach a 7th-9th grade YMCA team and had several girls doing this. I found the reason was because they were compensating for the fact that they hadn't developed the muscle memory to get their hands together quickly. I have them do a drill where they quickly adjust back and forth from a dig to a set position with their hands. Once they know that they can put their hands together in a split second in the correct position, they quit the bad habit of running with hands together. I like Coach Balasco's race suggestion as well.

If you are still having problems with this issue I will share a tip I picked up from another coach. Video tape the players and show them the video. When the players see themselves they understand how they look and self-correct. This works will most problem issues. Also show them via video the correct action.
Good Luck.