Network.maxConnections  

static var maxConnections : int

Description

Set the maximum amount of connections/players allowed.

Setting it to 0 means no new connections can be made but the existing ones stay connected. Setting it to -1 means the maximum connections count is set to the same number of current open connections. In that case, if a players drops then the slot is still open for him. This cannot be set higher than the connection count given in Network.InitializeServer.

function StartGameNow() {
    // Don't allow any more players
    Network.maxConnections = -1;
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void StartGameNow() {
        Network.maxConnections = -1;
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def StartGameNow():
        Network.maxConnections = (-1)