remove_all_variables.php

<?php
/**
 * @file
 * Install
 */
 
/**
 * Implements hook_uninstall().
 */
function MODULE_uninstall() {
  // Remove all variables.
  $variables = db_select('variable', 'v')
    ->fields('v', array('name'))
    ->condition('v.name', db_like('MODULE') . '%', 'LIKE')
    ->execute()
    ->fetchAllAssoc('name');

  if (!empty($variables)) {
    global $conf;
    $variables = array_keys($variables);
    // Delete.
    foreach ($variables as $variable_name) {
      db_delete('variable')->condition('name', $variable_name)->execute();
      unset($conf[$variable_name]);
    }
    // Cache clear.
    cache_clear_all('variables', 'cache_bootstrap');
  }
}