Reporting just make VAC (Valve Anti-Cheat system used in BO2) check reported player for any signs of modifiyng the game's process by dlls of cheats. Unfortunately, if a cheat's dll is not in VAC base, it won't ban the player. So the guy you had seen cheating and then met the day after could just been using a new cheat, which is not listed in VAC's "list of dlls".
Also, you will not get banned if you're not cheating, because the system is fully automised. If you don't use cheats which inject in the game's process with dlls you can't be banned.
For conclusion, I'd like to tell you that you shouldn't worry. Treyarch have made a great job to prevent hacking on PC. There's much less hackers than in MW3 without any doubt. I've never met a real aimbotter since the game's release. Reporting really works just the way it did in BO1, and no matter how good you play you won't get banned if you don't cheat.
Sorry fo my bad English, I'm from Russia.
It does absolutely nothing most of the time, kawz. Please don't lead people on to believe something that is clearly not true. I've seen multiple wallhackers (blatant ones) get away scot-free because the steam and black ops 2 report systems do nothing.
I asked for a moderator to look over one particularly obvious case of wallhacking (the guy even PMed me begging me not to report him, and eventually changed his name/privated his account), but I've yet to get a response.