How do I stop iTunes from automatically assigning a hollow star rating to podcasts?
I'm not aware of any way to turn this off. Perhaps you could work around this issue by giving the podcast "album" a rating of 3 stars?

Then your smart playlist for delete could check for 2 star rating. Your smart playlist for "unreated" would check for 3 star rating. Your smart playlist for "keeper" could be 4+ stars…