How do I report after a game? (Riot employee impersonation)

I was in a game when a Jayce began to flame. After I asked him to stop, I suggested that flaming is bannable and that I would report him. He responded by saying that he works for Riot, and he'd ensure that he wasn't banned while I was. I know for a fact that this is untrue, and I'm pretty sure that impersonating a riot employee is super bannable. (I have pictures of chat where he said he was a riot employee)