This behavior is present in 1/3 Diamond+ ranked games. This needs to be made zero tolerance.
Look at this example from my last game.
We were winning and had just taken an inhibitor. We wanted to group bottom. Graves didn't group, he got caught, and he died. The enemy team engaged on the four of us after, we killed three of them, they killed two of us, and we disengaged. Janna told him that he needs to group with us:
http://i.imgur.com/0kWPTls.jpg
So, Graves get extremely defensive and decides he's just going to stop playing, because his bad decision was criticized. He spent the next 3 minutes in the jungle farming, refusing to help us push. We managed to catch two enemy players out, including their jungle, and took Baron without him. After we took Baron, he ran directly into the enemy fountain and left the game:
We continued trying to win 4v5, then he rejoined and continued farming the jungle about 3 minutes later. We lost a 4v5 and the enemy pushed to end the game, while Graves continued farming in the jungle. The enemy team ended the game, and we asked them to report Graves. To which he responded 'lol for what? because i dc'd?'
This is fucking stupid. It's reaching the point where the game isn't even fun, because of these fragile douche bags being in virtually every moderately high rated game and making their team lose at the drop of a hat.
I've been playing at Diamond since season 1 (Platinum before Diamond existed) and I've had the pleasure of watching this behavior happen more and more often as each season progressed. This kind of behavior was basically unheard of prior to season 3, and now it's reached a point where it's almost in more games than not. It's happening so often that I barely even want to play anymore.
Enough is enough. This is not acceptable behavior and shouldn't be tolerated even once. The fact that this type of bullshit happens so often indicates that there is little, if any, punishment for it.
To make this post even more exciting, this particular player is not new to me. I've been playing with them and watching them act like this for the past two seasons; they're S4 Master/S5 Diamond. This means that they've been getting away with this behavior for over two years.
I actually submitted a support ticket with screenshots of this player doing this last season, because I'd been encountering them so often. Considering they're still doing it, it's obvious that the ticket was ignored. Why is this behavior not high (if not the highest) priority for putting an end to?

got caught, admitted he made a mistake, and got bullied?