Is it safe to delete default image/file styles?
You shouldn't remove the default styles (as mentionned), but you can alter them using hook_image_styles_alter

You can remove the options from selected forms using hook_form_alter (as mentionned).

I tried to find a generic approach and happily found this page.

This is how I applied the info:

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

  recursiveRemoval($form, 'thumbnail');
  recursiveRemoval($form, 'medium');
  recursiveRemoval($form, 'large');
  recursiveRemoval($form, 'square_thumbnail');

}

function recursiveRemoval(&amp;$array, $val) {

  if (is_array($array)) {

    foreach($array as $key =&gt; &amp;$arrayElement) {

      if(is_array($arrayElement)) {

        recursiveRemoval($arrayElement, $val);

      } else {

        if($arrayElement === $val) {

          unset($array[$key]);

        }

      }
    }
  }

}


N.B.: Use at your own discretion.