In the lobby, open that person's player card, scroll down to "Report", and choose "Cheating".
There is a difference between someone getting the good side of lag comp and cheating. I remember in blackops I was obviously getting the fun side of it one day and I knifed the whole team within 15 seconds. They all left because they assumed I was cheating when the reality was I was knifing them before they even realised I was there.
I also love the communities idea of skill level. You are either good at the game or not. If that's the case then pretty much everybody sucks at this game.