I imagine that it will work something like this senario.
Lets call the good player Manly and the reporter baby.
So manly and baby are put in a game together on a map, and right off the bat manly owns everyone, and little baby gets upset and starts crying over the mic for manly to stop killing him. Frustrated, he reports him for boosting, manly goes 30-0 and baby goes 4-52.
So the report is sent, and WILL, I guarantee, will be reviewed by a computer program that checks the player score, and who he killed, and how often. If it sees that manly ONLY killed player Z, then he could be banned on the spot or sent to an employee for further review. If it sees that the score is the only suspicious thing, but manly killed player 2 then 7 then 5 then 7 then 4 then 2 then 5 then 6 then3, and so on with no pattern, he probably won't be banned, but it may be sent to an employee for review. Baby will probably get a warning for abusing the reporting system.
Im also sure they will have a place to dispute bans. If you do and they find you DID boost, they will laugh at you and post it on youtube. If they find that no, you didn't and they were wrong, they will probably un-ban you and give you some nice CoD points as a "we're sorry we banned you" type thing.