The MAC address is "above" the IP address, mIRC does not deal with them so there are no built in functions to operate with MAC addresses in mIRC Script. The only possibility I can imagine is to use a function from an external DLL to get the MAC address of a specific IP address.
Now that I read your post again, do you mean banning from channels based on MAC addresses? The banning procedure is handled by the server, the client only sends the command for it. That means you need a server software that supports MAC-based banning. I've never heard of such a thing.