If that hacker is know the MAC Address that’s allowed to connect to the network, “You mention that you enabled MAC filtering" he can easily spoof his MAC Address and gain access to your network.
You have two choices.
1 Change your network card! Easiest and more secure.
2 Use an utility such as SMAC http://www.klcconsulting.net/smac/
If security is a real concern I’m sure he doesn’t mind to spend £30 Pounds on a new card.