ifconfig ethx down
where ethx is the interface. This command is to deactivate the ethernet card.
ifconfig ethx hw ether xx:xx:xx:xx:xx:xx
where xx:xx:xx:xx:xx:xx is the 12 digit hex address which you want the address of your ethernet card to be.
ifconfig ethx up
This command is to activate the ethernet card.
And you are there.
There are softwares like macchanger which can also be used to change the MAC address. They do not do anything different from the previous method expect allowing you change to MAC address from particular vendor. So, the above method is advisible rather than using macchanger.
But your MAC address gets restored once you restart your computer. Now how to this permanently. Well, there are different methods for this. You can write a script and add to your run-level to load it when computer boots. But I prefer a more easy and convenient method of editing the file /etc/network/interfaces.
In the above file, you will see line like
Below this line add the following line:
hwaddress ether xx:xx:xx:xx:xx:xx
N.B. : You will need root permissions to do the above tasks.
The above examples are given with reference to Kubuntu-7.04. The methods described above should work with others Linux distros as well.