You cannot ban by MAC. Do not even think about it! It can't be done, and even if it could be done, it wouldn't work anyway.
What's MAC stand for?
Mmmm, couldn't find a good page explaining what are MAC addresses without too much technical details. There is a good page on wikipedia but with even more details than I cared for

MAC addresses are somewhat like IP but for the Ethernet protocol ( or for other too ). The basic idea is that each network card produced is assigned a MAC address at build time that doesn't change and that is unique. It is accomplished because the first part of the MAC address is assigned specificaly for those who build them ( you have some for IBM, some for Netgear etc ... )
There are many problems with that. First, MAC is purely an Ethernet tool. MAC addresses can only be read on a simple LAN because any router/switch will mask the MAC addresses of the network cards connected to it. The second is that despite the MAC address beeing assigned at build time, it is incredibly easy to change it.
For example, on windows XP, you can simply go to the device manager, open my network card properties and change it there. I'm sure it won't even need a reboot

Not all cards can do it but nearly all do, and most often when they can't, it is a driver issue I would say.