Befriended me then threatened me

Hi guys, I had a game which the opponent bot lane lost horribly. You know guys the usually happaned after that: Opponent ADC: WOW! Congrats you won a normal game, be in diamond and then come and see me..blablabla...Just the usual. After the end at the summary i said to the opponent carry: Hey take your C- vitamin pls. Then i quit the summary screen. A few sec later he befriended me, i was like why not lets see what come backs he has, so this happaned: Me: Did you take your C- vitamin? (Jokingly) HE/SHE/Something: Where are you come from i wanna slice your throat. Lets see your IP address, i hope your family die horribly and so on, and he left. I was like HOLY SH*T what just happaned here, i usually let it go but this time i couldnt. There is a level where a player can go and cannot. I just want to ask where can i report this behaviour because this type of player/s should get punishments. I have his name and part of the conversation. Couldnt print screen it all because after he deleted me (such a pity he was such a kind guy) of his/her/something's friendlist the chat window closed. So where can i officially report this kind of behaviour? Kind regards, Wthunder
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