Deleting comments is of no consequence
Did you know guys that even if someone deleted a comment in a post, it still is visible on their Activity stream? Isn't that a bug?
Click edit on your post, delete the text, save the empty post. Then you can delete the whole thing, and no text should show up on your stream (though the event will still be there). Thinking twice before posting in the first place might be the best policy though. ;)
You cannot delete the posts of others as a regular user.