Doesn't Black Ops support private servers, though? Which means there are/can be admins present to remove cheaters/hackers/glitchers?
Anyways, I have experienced similar stuff like Ripuli aswell; admins removing players from the game who are obviously better than themselves. In a game with killcams (like the one from MW2) you can see if people cheat/hack/glitch quite well, I think. If it displays where you are and your enemy is constantly aiming at that spot through a wall, he's most likely using an aimbot or wallhack ...
I have played with people in the past who are really good at hiding the fact that they have a wallhack though. One guy was even in my clan, played in several tournies and never got banned. We kicked him out of the clan after he stopped hiding it in a private match while trying to turn it into a joke, getting headshots with a sniper through boxes over and over, and stuff like that. Point is, even in a situation like a tournament, with everyone looking for cheaters, he was able to hide it.
This is where anti-hack tools are needed, when mods alone cant tell the difference between someone who is good at the game vs someone who is good at hiding that they cheat.