Unfortunately it's only possible to remove if no items have been sold from the listing. I found this out after mistakenly having it ticked on a few of my listings (I make custom listings private usually and it sometimes becomes the default setting when I go to list new items ).

If no items have sold from a listing, there should be an option to untick it near the postage and payments section when you revise. If you don't see it there, try clicking tje add or remove options link. You should also ne able to edit the applicable listings in bulk via selling manager.