Never going to happen.
However, put in a two step penalty:
-1. For a second offense in a week (first offense is forgiven, as stuff comes up sometimes), tag the quitter(s) with the entire team's LP loss as a penalty.
For example, in your game (listing the quitters last, and assuming this is their second or more), if the LP losses would have been 17, 15, 19, 21, and 20, they are instead 17, 15, 92, 92, and 92.
-2. For another offense within a week of the above, give them the LP loss above, and a ranked restriction for 20% of the LP loss, rounded up. (For example, the ranked restriction for the three quitters above, if it was a third offense, would be 92*.2=18.4 rounded up to 19 games each.)
Additionally, during the ranked restriction, the punishment clock is stopped. Meaning only after the full 19 games are won does the week clock start for clearing the possibility of receiving another ranked restriction.
EDIT: Additionally, make a game "safe to leave" for anyone who does not have a first offense quit (that would be forgiven). Meaning if one person has quit for 5 minutes or more, and you have not quit a game in the last week, you may quit this game without using the first game "forgiveness" under this policy. (This would make quits doubly painful, as you could then not quit a game another has quit until a week has passed.)