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.

Yeah, some stupid little kid kept threatening to demote me the other day, if I left the lobby. Left it, and nothing happened, so I'd say no. (This was on MW2 though).