Disable link on username in comments? How can I disable the link on authenticated an authenticated user's name in the comment submission form in Drupal 6?
I just tested this out, not sure if it is the best way, but it works.  Create a really simple custom module and implement hook_form_FORM_ID_alter().
YOURMODULE.info
name = YOURMODULE
description = Description of YOURMODULE goes here...
core = 6.x

YOURMODULE.module
&lt;?php
/**
 * Implementation of hook_form_FORM_ID_alter()
 */
function YOURMODULE_form_comment_form_alter(&amp;$form, &amp;$form_state) {
  $form['_author']['#value'] = $form['author']['#value'];
}

Place these files in a directory named YOURMODULE, inside contrib modules directory. Then enable the module.
(Be sure to replace YOURMODULE with the name you choose to give to the module everywhere it occurs.)