That's not a problem unique to Riven, although it does affect her more. FotM mains drag down the mains of all champs unfortunate enough to catch their eye. How can the average player discern a Riven main from a "Riven main"? You can't, not without doing some detective work first. And no one is actually going to look up if that toxic Riven was a genuine main or not; best just to chalk it up to a Riven main being a Riven main and move on.
That's not a fair attitude to have, but since when has human rationale been fair?
If you want to change that attitude, it's not going to be a short walk. One idea is that you could police your own. Get a bunch of Riven mains together, and make it known that real Riven mains don't AFK, don't flame, don't be salty, etc. In fact, real Riven mains go out of their way to be nice and help their teams. Really earn those honors after a match. It'll take some time, but that kind of movement can really catch on.