How to report a player throwing on purpose?

Is this even possible? Had a jungle briar in an emerald lobby in the infamous SEA server who was apparently a master players on a smurf. They got fed early, got a bunch of kills. We were basically winning the game but all of a sudden this player got tilted out of their mind because of some chat message. Their response in this situation was to throw the game but refusing to teamfight and permanently split as jungler, refusing to group for baron and other soft ints. They did not run it down, but we're not participating in fights and ignoring objectives. Them having all the kills in our team meant we lost, especially when enemy team got free nash because they were rage-splitting bot

My question is, how do I report such player? Does the usual reporting system even take into account soft inting like this? Especially because they're a smurf, even if they get banned for a few games, they'll just hop on their main and continue playing. So how do I report such behaviour