The only thing that will stop it is not showing the "in-progress" anymore. Take the option out and filter people into dungeons as spots are opened like every other game on the planet does. This is precisely the reason Blizzard took it back out when they thought it would be a good idea - people fishing to in-progress runs (and by people, I mean tanks) and withdrawing from fresh ones. Wanting to punish people for using a system is the wrong approach. Fixing the system is the correct approach.