Thanks Tom
Actually this does the trick:
/* Media: remove default image sizes
---------------------------------------------------------------------------- */
function themeprefix_remove_default_images( $sizes ) {
//unset( $sizes['small']); // 150px
//unset( $sizes['medium']); // 300px
unset( $sizes['medium_large']); // 768px
//unset( $sizes['large']); // 1024px
return $sizes;
}
add_filter( 'intermediate_image_sizes_advanced', 'themeprefix_remove_default_images' );