#disabled_options in 'checkboxes' element

Drupal core still doesn't have it... There are issues from 2006 requesting this.

Make it. Use https://gist.github.com/rudiedirkx/6200160 That seems to work.

<?php
$form['test_no_fs_roles'] = array(
	'#type' => 'checkboxes',
	'#title' => 'Roles 2',
	'#options' => drupal_map_assoc(array('a', 'b', 'c', 'd')),
	'#default_value' => variable_get('test_no_fs_roles', array('c')),
		'#disabled_options' => array('b'), // That's it!
);
?>