Require a permission to see signs, item frames and painting.

SignPermission.java

package com.comphenix.example;

import org.bukkit.Location;
import org.bukkit.Material;
import org.bukkit.entity.ItemFrame;
import org.bukkit.plugin.java.JavaPlugin;

import com.comphenix.protocol.Packets;
import com.comphenix.protocol.ProtocolLibrary;
import com.comphenix.protocol.ProtocolManager;
import com.comphenix.protocol.events.ConnectionSide;
import com.comphenix.protocol.events.PacketAdapter;
import com.comphenix.protocol.events.PacketEvent;

public class SignPermission extends JavaPlugin {
    @Override
    public void onEnable() {
        final ProtocolManager manager = ProtocolLibrary.getProtocolManager();
        
        manager.addPacketListener(
            new PacketAdapter(this, ConnectionSide.SERVER_SIDE, 
                    Packets.Server.VEHICLE_SPAWN, 
                    Packets.Server.ENTITY_METADATA) {
                @Override
                public void onPacketSending(PacketEvent event) {
                    if (!event.getPlayer().hasPermission("cansee.itemframe")) {
                        // Handle the spawn packet and the metadata packet
                        if (event.getPacketID() == Packets.Server.VEHICLE_SPAWN) {
                            Packet17SpawnObjectVehicle spawnFrame = new Packet17SpawnObjectVehicle(event.getPacket());
                            
                            // Use getX(), getY() and getZ() to get the coordinates.
                            if (spawnFrame.getType() == Packet17SpawnObjectVehicle.ObjectTypes.ITEM_FRAME) {
                                // Excellent - disable it
                                event.setCancelled(true);
                            }
                            
                        // The packet that updates the content of the item frame
                        } else {
                            Packet28EntityMetadata metadata = new Packet28EntityMetadata(event.getPacket());
                            
                            if (metadata.getEntity(event) instanceof ItemFrame) {
                                event.setCancelled(true);
                            }
                        }
                    }
                }
            });
        
        manager.addPacketListener(
            new PacketAdapter(this, ConnectionSide.SERVER_SIDE, 
                        Packets.Server.ENTITY_PAINTING) {
                    @Override
                    public void onPacketSending(PacketEvent event) {
                        if (!event.getPlayer().hasPermission("cansee.painting")) {
                            //Packet19SpawnPainting spawnPainting = new Packet19SpawnPainting(event.getPacket());
                            
                            // We'll just cancel it all - but you can get the coordindates of the painting
                            event.setCancelled(true);
                        }
                    }
                }
            );
        
        manager.addPacketListener(
            new PacketAdapter(this, ConnectionSide.SERVER_SIDE, 
                        Packets.Server.UPDATE_SIGN) {
                    @Override
                    public void onPacketSending(PacketEvent event) {
                        if (!event.getPlayer().hasPermission("cansee.sign")) {
                            Packet82UpdateSign spawnPainting = new Packet82UpdateSign(event.getPacket());
                            Location loc = new Location(
                                            event.getPlayer().getWorld(), 
                                            spawnPainting.getX(), spawnPainting.getY(), spawnPainting.getZ());
                            
                            // Don't update the sign
                            event.setCancelled(true);
                            
                            // Send the fake block update
                            event.getPlayer().sendBlockChange(loc, Material.AIR, (byte)0);
                        }
                    }
                }
            );
    }
}

Well, I'm a little late (around three months late) but, you need to reference the PacketWrapper package in your project to be able to access the simplifies packet classes.