I'm reminded of the times, watching some of my fav streamers (during champ select/banning phase). When someone would threaten to troll/feed, if they didn't get their specific role (sometimes, they'd do it because their fav champ was banned).
Many times, the troll would try to blackmail people by demanding a free skin.. otherwise they'll troll/feed.
Then the streamer would point out the fact that the match was being recorded and that if he did indeed feed/troll, that the video would be sent in with a report.
This would either result in the person acting like they were just "joking".. while other times, they just dodge or infact actually do go in and feed/troll, operating under the assumption that the video evidence wont mean anything.
IE: So what? I won't get banned anyways!
Honestly, I think a built-in feature for replays (that records chat somehow) would go a long way to improving the state of toxic players in the game.