It's just a matter of hiding the magnifying glass icon, which is simple, with some custom CSS:
.fbx .fbResizer {display:none;}
ETA: The catch, of course, is that without that little icon, there's nothing to tell a visitor that he can zoom on the image. Accidentally tapping or pinch-zooming instead of swiping will reveal that fact - that might be good enough.
In fact, upon reflection, I might remove the
Zoom on phones checkbox entirely, and just have that
always active, and in keeping with my "get rid of screen clutter" approach,
never show the magnifier icon. Opinions?