While you can't 100% get rid of dumb players you sure as hell can take measures that prevent people who have no business there from getting in.
There needs to be a set number of games or mastery points in a champion before you can touch them in ranked as well as X amount of days owned. Idc if that's "unfun" for PBE players. People coming into my ranked games with 0 mastery points asking me what to build or what rune/ability they should take have no logical reason for being allowed to use them in ranked.
Similar to overwatch we need a notification saying what we're missing and what would be a RECOMMENDED pick. If our top laner is teemo and our support is a nami then don't pick a shaco jungle. We need a tank or some sort of front line not another squishy bursty champion. Nobody would complain about this. Idc if it makes people feel like you're enforcing anything. People. Are. Stupid. Point blank. You can't stop this, nobody can. So you need to take precautions against them if you expect people to stop screwing your game up.
Lock peoples screens into the game for the first few minutes while camps are spawning. The amount of games i've played where someone will run into tri bush after shopping then afk to alt tab then dying is so irritating. Invades are literally ONLY successful if someone isn't paying attention to the game, otherwise they wouldn't be a thing. Not that I think invades are bad, just the stupid people who refuse to pay attention are.
I could go on forever but please do something to keep people who have no business being in ranked out of it. Some of us are actually trying to win and trying to be decent people while doing it but when you put us in game with a ivern mid building ad vs a zed we are gonna start getting pissed off because the blame isn't entirely on them. You share a huge part of it as well