What did you try to solve this problem ?
Did you understand where this comes from ? Look at your events. As you describe it, "When click on red ball and drag over the yellow ball, both balls are thrown". This means that the "trowing" event is also triggered when the mouse goes on top of the yellow ball.
So if you want the mouse to trigger only the first ball it touches during clicking, you need not to activate the "ivPickedBall" of each ball every tick. Instead, you need to activate only the first ball touched, and then set this boolean to false on touch end.