Is there a way to delete/purge all generated image style images without re-generating them?
You just have to do drush image-flush --all. Source : http://www.drushcommands.com/drush-6x/image/image-flush

Examples of using the drush image-flush command:


drush image-flush - Pick an image style and then delete its images.
drush image-flush thumbnail - 'Delete all thumbnail images.
drush image-flush --all - 'Flush all derived images. They will be regenerated on the fly.'