OK, so I gave it a shot, and hiding the “on” was a real challenge, because it has no selector or tag…
Add the rules below in Advanced Options > Head Section > Custom CSS Rule box should do it.
.comment-author {visibility:hidden;}
.comment-author .fn, .comment-author img {visibility:visible;}
The first one makes all the author line hidden which allows the “on” to disappear, and the second one brings back the two pieces we want, which is the image and the name that fortunately has a class fn