How to report someone out of game.

Someone decided to be super toxic in our game, intentionally feeding, but luckily the patch came through and we all disconnected. However, I still believe he should be reported, but because we all disconnected, I could not use the endgame reporting system. I also put a post up but I believe it got deleted. I want to do this correctly to make sure this person does not ruin anyone else's games. Where can I go to report this person?