How to remove link of username in comment submission form
Simple solution is to use CSS. Just add a css rule to your theme's stylesheet.

#some .rule {display: none;}

Alternatively you can use hook_form_alter to modify the form and remove the link or change it to something else.

How to do it depends on your Drupal verion, see the API docs for reference or see altering form with hook_form_FORM_ID_Alter for usage on hook_form_FORM_ID_alter which is a specialized version of hook_form_alter.