How do I report a player if I met a troll in the champ select ?

I had uploaded a picture to this board, about the troll player, but Riot deleted it and said to use the in-game reporting system. But according to my knowledge, I can only report players at the end of the game. Then if I see a player trolls even in the team selection, in my case I met Sona jungle with no smite, how should I report him? Can someone tell me or Riot please tell me?