Ok, In this guide I will teach you the very basic steps of removing your CoD4 GUID making you invincible to their attempts to ban you.
The requirements for this is to have Cheat Engine installed.
1. Open CoD4 Multiplayer
2. Open Cheat Engine
3. On Cheat Engine, click the button "Add Address Manually"
4. In the box enter this: 006D04F8
5. Click OK.
6. Change the value of the address to "0" (zero)
7. Click OK
8. Join a CoD4 server
ONLY WORKS ON CRACKED SERVERS!