How to make a 3D ball stop rolling as soon as you are done pressing the assigned button

I am trying to make a ball rolling game that has the ball roll along the x-axis and z-axis with jumping. I can make the ball roll either axis, but it becomes difficult to maneuver the ball back from the z-axis and roll it on the x-axis, and vise versa. What I need is a code that makes the ball stop rolling as soon as I press, say the A key, on the respective axis

Here is my code

 #pragma strict
 
 var speed : float = 5;
 var rotationSpeed : float = 100;
 var jumpHeight = 8; 
 
 private var isFalling = false;
 
 function Update ()
 {
     //Handle ball rotation.
     var rotation : float = Input.GetAxis ("Horizontal") * rotationSpeed;
     rotation *= Time.deltaTime;
     rigidbody.AddRelativeTorque (Vector3.back * rotation);
     var rotation2 : float = Input.GetAxis ("Vertical") * rotationSpeed;
     rotation2 *= Time.deltaTime;
     rigidbody.AddRelativeTorque (Vector3.left * rotation2); 
     
     if (Input.GetKeyDown(KeyCode.Space) && isFalling == false)
     {
         rigidbody.velocity.y = jumpHeight;
     }
     isFalling = true;
 }
 
 function OnCollisionStay ()
 {
     isFalling = false;
 }