It is very difficult to properly monitor and then punish players that get reported without someone filtering the bans. You got to think that Riot probably gets thousands of bans every day and to go through those is really hard. So when someone keeps getting reports and they gather a large number of them, they are able to identify that person as a problem and punish them. Another thing is the automatic checks, the 'Bad' word bans as you see a lot of. Chat related bans are the easiest to monitor because they can go through an automatic system. Player and gameplay related bans take time to go through. If we had someone like the tribunal in place again that went through player reports again, then maybe, MAYBE we could see players like this get banned.
One thing that helps for going to report someone is fill in little box with information on what the player was doing. IF you fill that out it'll be easier for a Riot to make a call if they see a player with a number of reports. It gives him insight into what may have happened and what the people reporting him were thinking.