How can we get phyics so you can run down mail boxes, poles, etc with a car, but not with a human?
And can the script of these items come over with prefab?
You can set the objects with colliders and rigidbodies and mark the player within the Physics simullator to be ignored by those objects.
Whatever you include in an object before making it a prefab will be available after its creation. Actually, I´m not sure of everything, but scripts will be for sure.
Thanks 
So I have to copy/paste it, or I can still drag the object from below?
Or, you can use joints on the base of the mailboxes and just increase the break force to a value higher than a human object could impart.
Just drag like you always did. Marker16´s suggestion is a good one. Try it too.
I didn’t knew about joints/break force (witch is exactly what I was looking for). Are they integrated in Unity or we have to do something fancy to set them up ?
Also… another question, the game (hopefully) will be multiplayer, and I have doubts about using a destructible environment for the game. Aren’t physics the networking killers ???
Thanks for your replies 
About joints, just read the documentation, it´s everything there. The real network killer is excess of messages being exchange between the peers. Even with a good physical simulator, you´ll have to write your own code to synchronize the simulation between all the players. Depending on what you have in mind, it´s not trivial, and that Unity doesn´t have for you with just a drag and drop. Search “Dead Recknoning” on the internet.