Main.php

<?php

namespace dktapps\NoRak;

use pocketmine\plugin\PluginBase;
use pocketmine\command\CommandSender;
use pocketmine\command\Command;
use pocketmine\event\Listener;
use pocketmine\event\server\NetworkInterfaceRegisterEvent;
use pocketmine\network\mcpe\RakLibInterface;

class Main extends PluginBase implements Listener{

	public function onEnable(){
		$this->getServer()->getPluginManager()->registerEvents($this, $this);
	}

	public function onNetworkInterfaceRegistered(NetworkInterfaceRegisterEvent $event){
		var_dump("hi");
		if($event->getInterface() instanceof RakLibInterface){
			$this->getLogger()->notice("RakLib is not registered!");
			$event->setCancelled();
		}
	}
}

plugin.yml

name: NoRak
main: dktapps\NoRak\Main
version: 0.0.1
api: 3.0.0-ALPHA7
load: STARTUP
author: dktapps
description: Skeleton plugin generated by PocketMine-MP DevTools
website: https://github.com/pmmp/PocketMine-DevTools

I recommend to use ScriptPlugin for tests :-)

I generally do, but in this case it was quicker to just generate a skeleton plugin with devtools.