Proposed idea: Instead of banning or punishing people, how about we fix the root cause of the problem: People who check join in progress should not get matched with brand new parties.

don't display any progress on the DF, that will fix this issue
No, it won't fix this issue. Instead it will make people not use join in progress and then people who lose members mid run won't be able to replace them. Why decieve players, when you can simply make it so that people who join in progress only get into actual in progress runs? It's safe to say that if you check that box, you DON'T WANT a brand new party.