Some bans are not bypassable by changing Cd-Key. Either they ban the first IP number (92, 89, 74, 121, etc) or did a hardware ban.
On some servers, if you change/spoof your Cd-Key to a valid one, a message will appear saying "Invalid CD-Key". There is proabably another variable number group that it checks too to see if that is in-range.
You can always try block that message, but you will need an undetected packet editor first...