FORM API - checkboxes

$options = array(
  'poison' => t('Sprays deadly poison'),
  'metal' => t('Can bite/claw through metal'),
  'deadly' => t('Killed previous owner')
);

$form['danger'] = array(
  '#title' => t('Special conditions'),
  '#type' => 'radios',
  '#description' => (t('Please note if any of these conditions apply to your pet.')),
  '#options' => $options,
  '#weight' => 25
);

This is the code that i would like to implement in my form HOWEVER how would i disable (ie grey out) the "metal" in my list of options?

It seems to me the FORM API is missing this functionality.

Any advice would be sweet!

Jon...