MOBA toxicity is a design problem. These players are like this because these games are like this.

If good players wading in among the noobs is such a huge problem, what have you done in the last twenty years to make it stop being a problem? Stopping it from happening is a naive band-aid. A kneejerk response to the symptoms of systemic issues. This game has one hundred and twenty-four characters. Don't tell me the devs are unfamiliar with tweaking complex systems for macro-scale results. Low-skill games are where you'd expect handicaps or dynamic difficulty applied to all the numbers on either team. An e-sports demigod and a fresh player using DK bongos should be made similarly effective. If that disconnect between action and results frustrates the guy pushing 400 APM... good.

If you need a whole-ass reputation system just to identify people who play badly on purpose, why the fuck do you allow bad play to matter so much? Maybe lashing strangers together for an hour, when half of them will lose, is a potent source of bitter feelings. Maybe a game that punishes people for leaving when they feel bad isn't a recipe for positivity! But hey, at least they can keep their head down and mind their business, oops nevermind it's all interdependent teamwork. Well at least four good teammates can make up for one bastard... oh, did you never fix "feeding?"

Whoops.