Placing an object with limits

Hi there.

I've used the mouse object placement found on the unity coder blog in order to help me instantiate some game objects whenever I want to but I appear to have an issue.

Does anyone know how I could add to the below code something like a limit or bounds around the gameobject that is instantiated? I am asking this because whenever I click on the item that is instantiated it instantiates another one which it allows me to move instead of simply allowing me to move the first one.

Thanks, Eugen

 #pragma strict
  
 public var ObjectToPlace:Transform; 
 private var clone:Transform;
  
 function Update (){
  
 if(Input.GetButtonDown("Fire1"))
 {
 var ray = Camera.main.ScreenPointToRay (Input.mousePosition);
 var hit : RaycastHit;
 var layerMask = 1 << 8;
  
 
 if (Physics.Raycast (ray, hit, Mathf.Infinity, layerMask))
 {
 clone = hit.transform; 
  
 }else{ 
  
 layerMask = ~layerMask; 
 if (Physics.Raycast (ray, hit, Mathf.Infinity, layerMask))
 clone = Instantiate(ObjectToPlace, hit.point, Quaternion.identity);
 }
 }
  
 if(Input.GetButtonUp("Fire1")) 
 {
 clone=null; 
 }
  
 if(Input.GetButton("Fire1")) 
 {
 if (clone!=null) 
 {
 var ray2 = Camera.main.ScreenPointToRay (Input.mousePosition);
 var hit2 : RaycastHit;
 var layerMask2 = 1 << 8;
 layerMask2 = ~layerMask2;
 if (Physics.Raycast (ray2, hit2, Mathf.Infinity, layerMask2)) 
 {
 clone.position = hit2.point;
 }
 }
 }
 }