Some penalty means nothing. Resetting their streak means nothing, because everyone on the leftover team will get their streak lowered as well due to the loss. Which promotes even more leaving, because why would anyone care that their streak is being removed if they have no streak? Simple solution is to remove the option to leave. Meaning that if that person really wants to leave, they have to quit the game entirely. Or alternatively, just don't lower the streak if someone left during your game. Because it's a shit mechanic that nobody can control