They could steal some ideas from League of Legends.
In LOL, if you leave you can not join until the game is over and get a penalty (like deserter flag) put on.
Also, you can report players for afk and if they get a bunch they could accrue a ban... would suggest 12,24,36,48 hours (1st, 2nd, 3rd, 4th offense).
Since most people will be playing 5v5, it might not end up being a big deal, most of the time.