Demoting doesn't work. If you get demoted (By a hacker. There is no way to get around an IW demote) just go back to the Main Menu then back to Multiplayer. It's happened to me 3 times in World at War, that's all I had to do and I got back to my regular level. If that doesn't work then IW always has a copy of your rank or something like that in the case that something like this happens. What I want to know is:
1) Why would he derank you in the first place?
2) Was this in a Public or Private Match? If it was public then report him.
For a while there I kept getting demoted when the game first came out. It was nearly impossible to get through the first prestige. I was demoted twice at zero prestige by just a few level, maybe 10 tops. Then after I prestiged I kept getting knocked back down to zero prestige
Not sure why this happened but after a couple days I never had this problem again