Delete all entities of a type

EFQ

$entity_type = 'node';
$bundle = 'article';
$query = new EntityFieldQuery();

$query->entityCondition('entity_type', $entity_type)
  ->entityCondition('bundle', $bundle);
$result = $query->execute();

if (isset($result[$entity_type])) {
  $ids = array_keys($result[$entity_type]);
  entity_delete_multiple($entity_type, $ids);
}