In older WP versions I was able to set the value "0" (zero) in the upload media settings to prevent thumbnail generation.
This doesn't seem to work anymore :|
"Zero" or "empty" are getting ignored and thumbnails are created in several sizes.
Might be ok if one has 100 or so images, but I am uploading more than 2k images. It takes forever and manually deleting the (for me useless) created thumbnail files (several MB) are just making me crazy :o