This suggestion has been posted before, but it needs to be suggested again until it is implemented.
There absolutely needs to be an option to delete played episodes from the episode list. Not everyone will use this, but many will, and it's considered basic functionality on other podcast software by now. The same option should exist for downloaded episodes.
If Spotify wants to compete with established podcast software, they need to at least match the basic feature set. This is not a "bells and whistles" kind of feature.