I Just Met The Worst Person Yet!

So, this Dr. Mundo is the rudest person I've met yet. It's getting way too much for me. I don't want to mute other players because we lose communication, but at the same time, I really want too! The Mundo didn't gank any lanes at all and flamed our botlane because we had ADC Yasuo (though it was the only lane he ganked, but it was ONE time). He refused to gank because we had a poor start and flamed us for every mistake, and even the enemy Talon wanted to report him. Why are people like this allowed? I just don't understand. Please, someone just tell me, why doesn't Riot just ban these people? Is it hard to find toxic players? Or is it because they want more money from these people. I just don't know, why do people flame at all anyways and why doesn't Riot ban these people?