How to Report a player outside of game

So i have a major problem and am getting tired of it, a lot of people are smurfing and trolling within rank games, for example a player on my team bans my friends champion and then picks my champing within a rank match which puts us within a bad situation and it basically forces us to dodge the game and we get time penalties, there most be a way to report people outside of the game and i cant seem to find a way this is like the 5th time today