Use this link:
http://www.callofduty.com/profile/preferences
Down there at GamerCard fill in a Space as GamerTag (don't leave it complete empty, the space must be there) and uncheck the option "Display as Forum Signature".
Then click on "Save Changes".
You'll get an error message which says: "There was an error retrieving your gamercard image. Please check your username and try again".
Ignore this message and click on "Save Changes" again.
Now you will get the message "You preferences have been updated" and it will be gone.