Stop image resizing in particular case – is that possible?

Use the Filter Hook intermediate_image_sizes_advanced for control the resize function for different sizes. This hook get a array of different sizes and can control your resize with your requirements. Add your logic inside a plugin and remove sizes via this hook.

 // the hook in core
 $sizes = apply_filters( 'intermediate_image_sizes_advanced', $sizes );

A small example to remove the resize for mediumsize.

add_filter( 'intermediate_image_sizes_advanced', 'fb_filter_image_sizes');
function fb_filter_image_sizes( $sizes) {

    unset( $sizes['thumbnail']);

    return $sizes;
}