343, take a page from Nintendo.

When someone leaves, automatically ban them from playing online for 10 minutes straight away. No “waiting for repeat offenses” or anything like that, just do it.
In Super Smash Bros, another game I play a lot, if you leave mid-game, you get the above penalty right away. In my opinion, this would really help the people on Arena who just leave in the middle of a game because it wasn’t going their way. I just got done with yet another game where my teammates ran away because we weren’t winning. If you want to stop this, make the punishments more severe and sooner rather than later.