Hi there,
Thanks so much for using Pocono.
I’m sorry to tell you that this is not possible. The theme uses a different layout and HTML markup based on whether a featured image is assigned or not, so it is not possible to just hide it with CSS code.
If you don’t want a featured image on single posts, you have to remove the featured image from the whole post. But then there won’t be an image on the archives as well. That’s just how the theme works.
Cheers,
Thomas