permanently change the MAC address on a card. It is burned into the hardware chip. At most you might be able to have the OS do it for you each time it wakes up, as windows will with the registry, but I don't know off the top of my head how to do that on OS X.
However, this highlights a bigger issue. You should not have multiple devices on a network with the same MAC address. You should replace an entry in the table with the MAC address of the wifi-adapter.
I'm not sure of any routers that actually limit the number of MACs you can have on a wireless network. Are you talking about MAC-address filtering? If that's the case, then I wouldn't really bother with the filtering. You yourself are proving just how easy it is to circumvent. It adds a single layer of complexity, but no additional security (anyone trying to break into the wireless network is going to listen for a MAC that the router responds to and clone that MAC as the first thing they do).