Hi there, due to the way that the HTML is structured in Natural, there isn’t a way to hide just the “by” and the author. The “Posted on April 13, 2017 by” is a single string of text created by the theme software, so we can’t remove just the “by”.
I have filed an enhancement ticket with the theme author to have the HTML restructured so that the date or author can be hidden separately, but this will be the author’s decision. Hopefully they will look at things and make the change. I’ll post back here when I get a response from them.