It IS unacceptable.
I've never had something like that, though. The closest was this girl who liked to talk dirty. She got the boot not too long after the first message she sent me.
I'd say Call of Duty 4, since a lot of people still play it. You can also try Call of duty: world at war. A lot of people still play that as well, and will continue to do so for a while.
It's a cheap way on getting kills. Aside from ruining the game experience for most of the other players, it shows one of two things about the camper. Either:
-They only care about their stats (k/d ratio, etc) or,
-They aren't good enough to go out and kill people face to face, thus forcing them...