Riven has had years of poor PR. As have a couple other champs. If you want to turn that around, you're going to have to do things that improve the image of the players that main that champion.
And no, this isn't just being a saint while playing Riven or saying stuff like "Hey, guys! I'm a Riven main that isn't ragequitting right now!". You're going to have to do some image repair. Maybe start streaming or unloading vids to youtube and be a positive influence on the community. BoxBox can only do so much by himself, and even he slips up and gets salty from time to time. Getting salty once in a while is fine for most other champ mains, but everyone is going to be more critical of a Riven main's behavior. You can't afford slip ups like that.
Worst comes to worst, do what the big companies do when they need to repair their image; bribe people. Everyone loves RP giveaways. A few years ago there was a thread that you had to post something creative in about Christmas and League, and five or six posters were randomly chosen and got sent RP codes. The same guy did these type of giveaways for three or four years in a row, and he was always welcomed to any threads he appeared in.