Just had the same problem.
Given:
class MyModel(Model):
image = ThumbnailerImageField()
You can delete all thumbnails with:
for m in MyModel.objects.all():
m.image.delete_thumbnails()
If you instead have:
class MyModel(Model):
image = ImageField()
Then you should use:
from easy_thumbnails.files import get_thumbnailer
for m in MyModel.objects.all():
thumbnailer = get_thumbnailer(m.image)
thumbnailer.delete_thumbnails()