add in a boolean value to see if you've already clicked on it or not. Something like:

var activelyRot : Boolean = false;

function Update () {
    if(Input.GetMouseButton(0)) 
        activelyRot = !activelyRot;

    if(activelyRot){
        xDeg -= Input.GetAxis("Mouse X") * speed * friction;
        yDeg -= Input.GetAxis("Mouse Y") * speed * friction;
        fromRotation = transform.rotation;
        toRotation = Quaternion.Euler(yDeg,xDeg,0);
        transform.rotation = Quaternion.Lerp(fromRotation,toRotation,
                                  Time.deltaTime  * lerpSpeed);
    }
}

This should let you click once to begin controlling it, then click again to stop it.

Not sure why, but I'm getting a compiling error when I run this. complete code:

var speed : int;
var friction : float;
var lerpSpeed : float;
private var xDeg : float;
private var yDeg : float;
private var fromRotation : Quaternion;
private var toRotation : Quaternion;
var activelyRot : Boolean = false;

function Update () {
    if(Input.GetMouseButton(0)) 
        activelyRot = !activelyRot;

    if(activelyRot){
        xDeg -= Input.GetAxis("Mouse X") * speed * friction;
        yDeg -= Input.GetAxis("Mouse Y") * speed * friction;
        fromRotation = transform.rotation;
        toRotation = Quaternion.Euler(yDeg,xDeg,0);
        transform.rotation = Quaternion.Lerp(fromRotation,toRotation,Time.deltaTime  * lerpSpeed);
    }
}