Feeds import - Skip importing item based on some condition

You can do this using hook_feeds_presave to check the data prior to it being saved.

$item  parameter contains the current feed. 
Check item on some condition.

To skip an item to being import, use $entity->feeds_item->skip = TRUE;

function mymodule_feeds_presave(FeedsSource $source, $entity, $item) {

  // check that this is fired only for the importer being used
  if($source->importer->id=='my_custom_feeds_importer'){

    if($item['mfgcode'] < '18'){
      $entity->feeds_item->skip = TRUE;
      drupal_set_message(t("Skipping item", 'warning');
    }
  }
}