How do you block a player who you did not play a game with, but was only in the lobby with?

I was in a lobby and encountered a toxic player but thankfully someone dodged before the game started. It's probably a good idea to block them (I'm hoping Riot's algorithm won't match me with people who I block). I remember in the old client, that was possible, but now I can't find that option in the new client. Anyone know?