Why isn't smurfing banned?

I just lose a game in s3, while having plat+ top Riven against me. I have Olaf 6 mastery and three tokens for lvl 7 and I haven't seen this quite a long time. Riven effortlesly killed me 9 times before lvl 15 (she just dived under tower dealing around 1k damage within few seconds on lvl 6). At this point it was clear I am not fighting against silver Riven. Why dont you ban these people or at least make them lose LP. This is outrageous.
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