How can I erase all photos on my iOS 11 device that can't be removed with Image Capture trick?
I found the answer and it requires some surgery by deleting the sqlite databases that the iPhone keeps for synced photos, the photos thumbnails and the photos themselves. I've documented the steps in a blog page here: http://dronefone.com/brian/blog/Entries/2018/3/7_Removing_Synced_Photos_from_iOS.html

Note this works for iOS 12 as well.