There is already a penalty for leaving: -75% experience gain. However, this is not discouraging for the behaviour itself, instead it discourages the appetite and fun of players to consume this gaming product.
The solution is simple, as previously stated multiple times already: fix the matchmaking rating of the game, so that matches may be more balanced, more entertaining, more engaging, more encouraging and welcoming towards play. Leavers will then have a reason to stay, the very same reason which pulled them into this game to begin with: it’s called fun.