Fixed object direction while another object is rotating

Hi,

I have two different object, pink and blue, and they are in hierarchy. As below.

When I rotate pink object, blue one would rotate at same time. However I need the blue one fixed in vertical direction, as figure in bottom right, instead of totally follow pink object. How to modify mine code? Hope someone could give me some suggestion! thanks

 using UnityEngine;
 using System.Collections;
 
 
 public class FreezeMovement : MonoBehaviour
 {
     public bool FreezeZ = false;
     private Vector3 OriginPos;
     private Vector3 OriginAngle;
 
 
     void Start ()
     {
         OriginPos = transform.position;
     }
     
     
     void Update ()
     {
         Vector3 currentPos = transform.position;
         if(FreezeZ)
             currentPos.z = OriginPos.z;
         transform.position = currentPos;
         
     }
 
 
 }