Computer MAC Address

I need to get the MAC Address of the computer, or something that cannot change about that computer.

I am working on a more secure banning system, with everybody having dynamic ips, once the ip changes they can log back in. So I was going to try to find something about the computer using javascript or php and add it to the database when they ban the user.

If you know of something or have worked on something like this, Please help. If you have any clues that would be great.

Thank You,
Jordan Martin
Seraskier