Once you've reviewed the game(s) in the theater, click on the "Friends" button in the lower right on the main menu screen (multiplayer main menu). You'll see a tab for recently seen players. Find the player, click on them and click the button to view their player card. On there should be a button to report them.
I'm not near a machine with the game on it, so my instructions above might not be perfect, but hopefully they are close enough to get you there.
Also, keep in mind that some players out there are just sickly good. I've often been convinced that someone was a cheater then in theater realized that they were missing a lot of shots too. Or as a ghost I've been followed through a wall only to see in the theater that they were constantly careful when going around corners (not something that a wall hacker would be concerned with) and just happened to have an SR-whatever in the air when they killed me or saw a small piece of me running behind cover.