1. Delete this item from all your custom Lists
By swiping it off in "Shop" mode or setting item amount lower than zero in "Prepare" mode
2. Delete it from Master List
(Master List -> "Done" to hide the keyboard -> "Edit" -> Select item you want to delete -> "Delete" -> "Proceed")
Items that are still present in any of your Custom lists can not be deleted from your Favorites list.
How do you get to the "Master List" or "Favorites" list?
There seems to be no menu for Master List in iOS, and the above question has not been addressed.