Some guy was screaming at me all game, calling me shit-tier garbage cancer trash for most of a match for being hyperfocused and trying to support an entire overspecialized team in every fight, in every situation at all times no matter what my current situation was. Reported him after the game, and because supports are almost extinct in this game and the block/mute system doesn't seem to function, I got put in another game with him (he didn't seem to realize I was the same support as last game).
After that game, I get the notification that a player has been punished. I sit in the lobby for a few minutes and note that he did not come into the lobby at any point, and based on his match history, he has not played a single game since.
The report system works. I witnessed it work in real time, and it only took one. Don't let anyone try to lie to you that it doesn't.