To delete a Streams comment:
Navigate to the comments section of the shared email or task.
Hover over the comment you want to delete.
Click the More Options icon.
Select Delete from the menu list.
Click Delete Comment in the alert dialog that appears.

Notes:
If a user has commented on a Streams post you've created, you'll be able to delete that comment. However, if you've commented on a post by another user, you'll be unable to delete anyone else's comment but yours.