The below snippet will disable the delete button on the taxonomy term edit page if the term has any nodes assigned to it. It implements hook_form_FORM_ID_alter()

function hook_form_taxonomy_form_term_alter(&$form, &$form_state) {
  $query = db_select('taxonomy_index', 't');
  $query->condition('tid', $form_state['term']->tid, '=');
  $query->join('node', 'n', 't.nid = n.nid');

  $count = $query->countQuery()->execute()->fetchField();
  if ($count) {
    $form['actions']['delete']['#disabled'] = TRUE;
    $form['actions']['delete']['#suffix'] = "<span>$count nodes, cant delete</span>";
  }
}