I don't know if there are working spoofers that will work because i don't use they, but maybe this tutorial that i I found maybe can help you:
Call Of Duty: Modern Warfare 3 (BLUS30838) 1.21 Fix for 3.55 CEX/DEX Users by Starchild2k
As you all know there has been a new 1.21 update for Call Of Duty Modern Warfare 3. This has currently disabled people using CEX/DEX 3.55 CFW with the latest FckPSN from playing online. I decided to go ahead and release a PKG to fix this problem.
1. Download this ZIP File I created: http://www.mediafire.com/?00yhnu97fwi5tdp
(i have replaced this because the original link went down)
2. Delete all Call Of Duty Modern Warfare 3 Game Data
3. Install the 1.19 game update I included in the Zip File I created
4. Install the 1.21 game update I included in the Zip File I created
5. Enjoy online play on MW3!!! (Must have FckPSN to play online!!!)
Note: This is only for the US version (BLUS30838) !!!!