How to permanently delete contact with API?
You can pass the skip_undelete parameter to the API, ie.

$result = civicrm_api3('Contact', 'delete', array(
  'sequential' =&gt; 1,
  'id' =&gt; 9999999999999999999, // replace with your contact's Id.
  'skip_undelete' =&gt; 1,
));


Obviously a lot of care has to be taken over specifying the id to delete...