Why do they still bounce?

I've made things like you can see in many tutorials and forums:
- created Physic Material I called "`No bounce`"
- set `Dynamic Friction = 0` and `Static Friction = 0`
- created a cube and add a `Box collider` with `Material = "No Bounce"`
- set `Mass = 1`, `Drag = 0` and `Angular Drag = 0`
Now I add another cube for the ground, made it very large and added a `Box collider` with `Material = "No Bounce"`
I have 2 problems:
- when they collide, the cube bounces (whereas it shouldn't with my configuration)
- I've made a script and attached it to the cube, to change the velocity, and set it to `0` when there's a collision:

Code (CSharp):

1.     using UnityEngine;
2.  
3.     public class CubeProperties : MonoBehaviour
4.     {
5.         private Rigidbody _rb;
6.         private bool _landing;
7.  
8.         private void Start()
9.         {
10.             _rb = GetComponentInParent<Rigidbody>();
11.         }
12.  
13.         public void OnCollisionEnter(Collision collision)
14.         {
15.             Debug.Log("Collision");
16.             _landing = true;
17.         }
18.    
19.         public void FixedUpdate()
20.         {
21.             if (!_landing) {
22.                 return;
23.             }
24.             _rb.velocity = Vector3.zero;
25.             _landing = false;
26.         }  
27.     }

So at the first collision, I try to instant stop the cube with `_rb.velocity = Vector3.zero;`. But *changing velocity has no effect*, I dont understand why. I've tried with many value to see what happens... but nothing happened.
The only thing I can add, and it's working, is: `AddForce()` I tried to a negative value, but this doesn't work either.
What did I forgot?
Here's a video I hope this is easy to understand (and I hope I'm allowed to help with a video):

 

you set landing = false;, so the FixedUpdate part only runs once ?

 

If you watch the video you'll see it runs when it hits something, but once. This should give something like:
- "you hit something, you instant stop"
- Unity physics takes control again, and if it has to fall, it falls again or it goes to sleep in .5 s
- if it falls again, restart the loop

This is the "basic" code. So it should instant stop when it hits something with no rebound at all.