Smurfs throw games on purpose? No punisment?

I had a smurf last game that got WAY ahead then proceeded to farm the jungle all game do nothing else. Literally once he got ahead by a LARGE margin just farmed. He never group even 40 minutes in the game just farmed. He Items all static shiv and refused to do anything. The game was 4 v 5 and we lose. I checked his History same thing. I would like to call him out, but apparently you can not on the boards. Why is this allowed? he taunted us on his team all game saying get good report me I do not care it is my 4th account. Why does Riot allow this? You can Easily IP ban people. Instead you let them RUIN 50 minutes of our lives in a 4 v 5 we should've won.Everyone else did good as well, but when everyone get one lane a lead and they troll on purpose leading to a 4 v 5 it is hopeless.Why is this allowed and why not IP ban them?
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