Can you report someone if you haven't been in game with them?

If this was already posted somewhere, I apologize. I couldn't find it. Feel free to send me the link if I missed it. I was wondering the question above. There was someone in one of my recent match makes that was trolling. I get it now and then in games, but they were being ridiculous... He first banned my champ for no reason, and proceeded to continue to troll me. They also started being troll-like to other people in chat. I took the bullet for everyone and dodged the game because nobody wanted to play with them. I however was never in game with them and couldn't find a function to report them. I assume riot is able to pull chat logs and see they started acting that way without even being provoked. Is there anyway I can report them? If a mod replies, that'd be amazing. However any answer I can get will do. If there's no way to do it, I'll remember that for next time and just enter game and take the wasted time just so I can report them. Nobody should have to deal with a troll idiot if they don't have to. Thanks for taking the time to read!