You need to refresh the page for it to take effect, but it should remove the category option from the homepage dropdown:
Edit: I think mobile vs desktop may have thrown me a curveball on this one. I will investigate this further…
Yes. It’s a separate box on the desktop layout - which I should have picked up from your screenshot. My apologies. I think the best way is through adding the CSS @awesomerobot suggests. 
However, I think @awesomerobot may have understood what you’re looking for better than I did. 