You might run into trouble if you trigger the event by clicking on a ball object since that automatically picks the ball instance you clicked on, making it trickier to test the proximity of other instances. I would get around this by using a function. Something like:
Mouse: On Ball Clicked
Function: Call Function "Ball Clicked" (Ball.X, Ball.Y, Ball.AnimationFrame)
Function: On "Ball Clicked"
Set variable Range = Whatever the distances is between each ball
System: Pick Ball by evaluating (distance(function.Param(0),function.Param(1),Ball.X,Ball.Y) <= Range)
Ball: Ball frame = function.Param(2)
System: Ball.PickedCount = 7
Ball: Destroy Ball
If you have a hard time implementing this, please post your capx.