Submit callback for poll_cancel_form().
function poll_cancel($form, &$form_state) {
global $user;
$node = node_load($form['#nid']);
db_delete('poll_vote')
->condition('nid', $node->nid)
->condition($user->uid ? 'uid' : 'hostname', $user->uid ? $user->uid : ip_address())
->execute();
// Subtract from the votes.
db_update('poll_choice')
->expression('chvotes', 'chvotes - 1')
->condition('chid', $node->vote)
->execute();
unset($_SESSION['poll_vote'][$node->nid]);
drupal_set_message(t('Your vote was cancelled.'));
}