hook_form_alter #attributes and #disabled
Checkboxes are converted to their individual form elements in form_process_checkboxes (), so you'll need a process function:

// In the form alter...
$form['taxonomy_forums']['und']['#process'][] = 'MYMODULE_process_checkboxes';

function MYMODULE_process_checkboxes($element) {
  $element[334]['#attributes']['class'][] = 'test-class';
  $element[334]['#disabled'] = TRUE;
  return $element;
}