Adobe ACROBAT READER 7.0 Manual page 123

Deleting reply messages
If you delete a comment that has been replied to, only the "parent" comment is deleted.
Any replies to the deleted comment remain in the document but are no longer part of a
thread. These comments may become difficult to read because they are stacked. You may
want to view them in the Comments List.
In a browser-based review, you can delete your own comments and replies, but you
cannot delete others' replies, unless you are working offline.
To delete reply messages:
Do one of the following:
In the note pop-up window, choose Options > Delete Comment.
Select the reply in the Comments List, and then click the Trash icon in the Comments List
toolbar.