I was in a match now as Soraka and Braum started complaining because someone didn't trade a champion with him, the guy that had Corki felt offended and said he wouldn't play the match. He stayed all the game following us, doing nothing. At the end of the match his name wasn't red
Because the red comes from a certain amount of time of no movement or activity basically. Report him as AFK, and move on.
Submit a support ticket or report them for unsportsmanlike and explain what happened in the box. Trolling has a much more severe penalty than AFKing anyway.
The names being red isn't even a part of leaverbuster it is just an indicator that they were idle at the end of the game.
