Unparent object on collision?

Could you be more specific about what object is which? Without much knowledge of your objects though, the following code removes an object's parent:

 gameObject.transform.parent = null;

Put this in a script attached to the main object.

public GameObject theObjectToBeUnParented;
void OnCollisionEnter(Collision collision)
{
    theObjectToBeUnParented.transform.SetParent(null);
}

 var certainObject : Transform ;
 
 function OnTriggerEnter (col : Collider) {   
 
     if (col.gameObject.name == "anObjectTrigger") 
     { 
             certainObject.parent = null;     
      }
  
 }