Simples

package me.kerooker.noopenhopper;

import org.bukkit.Bukkit;
import org.bukkit.event.EventHandler;
import org.bukkit.event.EventPriority;
import org.bukkit.event.Listener;
import org.bukkit.event.inventory.InventoryOpenEvent;
import org.bukkit.event.inventory.InventoryType;
import org.bukkit.plugin.java.JavaPlugin;

public class NoOpen extends JavaPlugin implements Listener {

    @Override
    public void onEnable(){
        Bukkit.getPluginManager().registerEvents(this, this);
    }

    @EventHandler(priority=EventPriority.HIGHEST)
    public void onInventoryOpen(InventoryOpenEvent e){
        if (e.getInventory().getType().equals(InventoryType.ANVIL))e.setCancelled(true);
    }

}