Can't unset default zen settings in theme-settings.php sub-theme

I've created a fresh sub-theme starting from STARTERKIT on a new local Drupal 6.20 installation.
On subtheme theme-settings.php I just added:
unset($form['zen_block_editing']);
after:
unset($form['themedev']['zen_layout']); // We don't need to select the base stylesheet.

Cleaned the cache, reloaded page, but "zen_block_editing" checkbox is always there.

I've also tried to update zen to latest dev version but same result.

Searching the web I found this article that show the same thing I'm trying to do:
https://www.packtpub.com/article/creating-custom-themes-zen-drupal-6 [Modifying myzen's theme settings]

Is it a bug or I'm just doin' something wrong?

Thank you