Somebody's already posted a way of "killing" Live Photos by editing them and unselecting the "Live" button, but this only hides the video part as it can be re-enabled later (i.e. it's a non-destructive edit). I've just come up with a Mac Photos workflow that maybe useful for people wanting to permanently change Live Photos back to normal ones and discard the video part to free up storage space.
1. Using Photos on the Mac, export the photos you want to change (File>Export>Unmodified Original) to a folder
2. Delete the photos from Photos
3. Using Finder, in the folder you exported to you'll find two files for each photo: .jpg (the still) and .mov (the video). Delete all the .mov files
4. In Photos, use File>Import to import the remaining .jpg files back into your library
There may well be other, simpler and better ways of doing this - if so please post them here!
1. Using Photos on the Mac, export the photos you want to change (File>Export>Unmodified Original) to a folder
2. Delete the photos from Photos
3. Using Finder, in the folder you exported to you'll find two files for each photo: .jpg (the still) and .mov (the video). Delete all the .mov files
4. In Photos, use File>Import to import the remaining .jpg files back into your library
There may well be other, simpler and better ways of doing this - if so please post them here!