Remove in-built featured images

Installed Septera and want to use it, but I have a problem with the featured images. Where I have my own defined featured images it works fine. But where I don’t have a featured image set, the theme inserts one of it’s own (which is irrelevant to the post content).

How can I get rid of those theme-defined images?

Bear in mind that I have several years of content, so I can’t go through old posts to edit each one.