You cannot ban someone's MAC address. The IRC protocol doesn't provide that information (internally or externally) for bans or anything else. You can ban a range of IPs or hosts using the * wildcard. Example, a ban on *@11.22.33.* would ban everyone with an IP from 11.22.33.0 to 11.22.33.255. Typically, if someone is reseting their modem to get a new IP/host, they will get one that is similar (one number different).
-genius_at_work