Drupal appends "not verified" string to user's comment as below,

Not sure why it does so nor I am aware of making a comment as verified but the following trick worked for me to remove it.

Click the theme "settings" link in theme manager page.

In the "Toggle Display" settings uncheck "User verification status in comments" option.

This will work only for standard drupal installation, for drupalgardens site the theme manager page is different and there is no clean way of doing it. Check http://www.drupalgardens.com/content/remove-not-verified-text-comment for workarounds.

Thanks to tuxnani for the tip.