Added as an edit: Yup - he's done that. We now have http://news.ycombinator.com/user?id=M_Arrington spamming the site.
Clicking link on a post [EDIT: on a comment] presents the flag option. You can then flag the post [EDIT: flag the comment] that way, which is the closest thing to flagging a user.
That works, but so does clicking link.