I'm not 100% sure how reporting and banning works in games but I think it's something like this-
People choose to report someone, the report is sent to, in this case, to Blizzard. The best way to ban hackers, bots and such is in waves. Banning in waves means that Blizzard gets reports from players, and they look into it, if the report match up to the story, and they can see that the hacker is using a software to bend the gaming rules, they will put him on the banning list.
They do this until they get a big amount of hackers on their list. After a period of time, they press the ban button, and a lot of hackers will be banned at the same time. This is usally done because the hackers will have harder time noticing exactly what it was that banned them, and they get a harder time fixing the hacker software after it.