Currently, you can only report players through Steam for cheating, being offensive, threats, etc.
There needs to be a dedicated, cross-platform function to report leaving and AFK team members and a punishment system. Just putting players back into the game they left does not help. It, in fact, hurts their teammates because that player will not be replaced by another.
Either that or there needs to be a way to reward players on the leaver’s team for playing down a player and losing (so long as they were randomly matched with that player so that there isn’t an exploit to rewards).