How to make server authoritative (and block network.Instantiate from clients) ?

I have been looking all over on finding a way to block clients from sending Network.Instantiate(..) to the whole network.

I managed to block RPC calls by checking for the sender:

 [RPC]
 public void enterVehicle(NetworkMessageInfo info)
 {
     if (info.sender.ToString() == "0" || info.sender.ToString() == "-1")
     {

But I can't find how to do these same checks on Network.Instantiate() calls

I've also read about using the following:

 uLink.Network.isAuthoritativeServer = true;

to block rpc and Instantiate messages going between clients, but I can't seem to find the isAuthoritativeServer variable.

All in all, my question is; how do I check the sender of a Network.Instantiate call ?