Eh, I tend to mute them even after that. I've had more people say stuff like, "of course your bad, get off of league and die" or something related to that even after I sent "mb". When I play with a duo partner, I rarely see any flaming at all, but I see flaming a lot when I solo queue.
From personal experience, I would say that there are a LOT of toxic players in League, since I've run into one pretty much every single game, be it on my team or on the enemy team. I've had people from the enemy team ask us to report their teammate for verbal abuse, refusing to communicate, or whatever. Even in ARAM, I've had people be toxic, even though it's just meant to be a "fun" game. Do I think there are more good-natured players than toxic players in League? Of course. But I'm simply saying that there are a LOT of extremely toxic players in this game. To say that there aren't any or that there are very little amounts of toxicity in this game would be inaccurate, at the least.
I've had players on my team who have made bad mistakes (e.g. facechecking bush solo and blind, then getting ganked). I've raged before, but I tend not to type it out unless they keep repeating the same mistake. Then I ask them to stop doing that. I've had players ignore it, then rage themselves, and I've had players say "mb" and then they stop doing it. I always respond to someone who stops making a mistake with "ty" or "thanks".
I'm not the best player. Far from that. I'm not the "nicest" player, but I'm also not the most toxic player in the game. However, I'd rather see people put out constructive criticism rather than insulting, which the latter happens most often, unfortunately. Hell, if you check the forums, there are toxic people who are saying that they are right, and their teammates are wrong. Those are the people I tend to avoid, and I prefer to avoid.