Well, I know this sucks and you feel bad after hearing this in a game, but what you should do is just ignore him. He just wants attention. I've been playing for so much time and I've got into this kind of players almost every game. You don't get anything if you keep talking to a toxic person. Don't lower to their level. I know, maybe you want to explain yourself or even to apologize for not having a good match but it's not worth it. They will always keep flaming and attacking you because that's all they can do. Your time and your patience can be used for something else, don't waste them on someone who just wants to tilt you. As I saw in another post, a single report is enough to get Riot to look over a player. So the best way to lower this kind of behavior is to report them after the game and ignore everything related right after. At least that's what I'm doing since season 2. And yes, I'd like to know too how to block a player via the new client. I can't figure it out.