Disable rich text editor by default
There's a solution here http://www.only10types.com/2011/12/drupal-6-enabledisable-wysiwyg-editor.html

Basically, you need to alter this specific edit form. This is what I did for Drupal 7 (the blog post is for Drupal 6):

function MYMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {

  // Workaround to disable Wysiwyg just in this field.
  if ($form_id === 'FORM_ID_OF_EDIT_FORM') {
    $form['field_CUSTOM'][LANGUAGE_NONE][0]['#wysiwyg'] = FALSE;
  }
}


You may want to use $language->language instead of LANGUAGE_NONE.