Screenshot: https://gyazo.com/6f473dbd77bc745ca61707baa55fa37e

Code in text:

package me.aguy.anvilguitest;

import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.inventory.InventoryOpenEvent;
import org.bukkit.event.inventory.InventoryType;

public class PlayerListener implements Listener {

	public PlayerListener(EventHandle plugin) {
		plugin.getServer().getPluginManager().registerEvents(this, plugin);
	}

	@EventHandler
	public void onOpenInventory(InventoryOpenEvent e){
		if(e.getInventory().getType() == InventoryType.ANVIL)
		e.setCancelled(true);

	}

}