Network.OnPlayerDisconnected  

function OnPlayerDisconnected(player: NetworkPlayer) {
    Debug.Log("Clean up after player " +  player);
    Network.RemoveRPCs(player);
    Network.DestroyPlayerObjects(player);
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void OnPlayerDisconnected(NetworkPlayer player) {
        Debug.Log("Clean up after player " + player);
        Network.RemoveRPCs(player);
        Network.DestroyPlayerObjects(player);
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def OnPlayerDisconnected(player as NetworkPlayer):
        Debug.Log(('Clean up after player ' + player))
        Network.RemoveRPCs(player)
        Network.DestroyPlayerObjects(player)