You can edit/delete for 2 hours. You cannot delete a comment that's been replied to. Unless you somehow convince your respondents to delete their comments within your edit window, which never seems to work.
Yeah, I should have replied to your original message so that you could delete the follow-up. As sibling comment points out, you can't delete once someone replies to your message. I knew better, I just didn't think it all the way through. Anyway, now you know.
EDIT: or "edit | delete [–]", as the case may be.