Cleanup storage on the Android device

I listen to most podcasts via the Android app. When I mark a lesson as "studied" and press "sync", it disappears from the list of lessons in the app.

Today I have taken a look at the files stored on my device and noticed that the Chinesepod folder is one of the biggest: There are still hundreds of lessons and podcasts which I have marked as studied several months ago. As they are not accessible in the app anymore, they just waste memory on my device. I don't see how I can delete the studied ones manually without opening each file and checking whether I really have studied this one or not. Is there a way to remove all studied lesson from the device automatically? If not, please make this a feature request for further releases.

Thank you.