I usually just do /fullmute all, when I get into a casual game. - If I get into ranked, I say hi to my team, tells them the only thing they can use to communicate is pings, and if they spamping, they're completely muted.
Haven't had any flaming so far in 3 years, even after the game. People have added me after, and used the method later on themselves, because of the toxicity in LoL.