Open the
Add / Edit Standard Comment form.
From the
list on the left hand side of the form click on the code of the comment you
want to delete.
This
will automatically load the comment into the right hand side of the form.
Now click the Delete button.
The Delete Comment Dialog will ask you to confirm that you want to delete the comment, click the Yes button to continue.
The Add / Edit Comments form will update to remove the deleted comment.