Hacker News implemented a flag button for comments quite a while ago. You go the "link" next to the comment header, to go directly to the URL of the comment, and then a "flag" link will be exposed there if you have enough karma to flag. I flag comments from time to time if they meet criteria laid out by pg last year
http://news.ycombinator.com/item?id=2403696
when he wrote, "The problem has several components: comments that are (a) mean and/or (b) dumb that (c) get massively upvoted."
A comment being mean is a signal that the comment may be silently downloaded (and flagged as well) and a comment being dumb is at least a signal for a downvote (although in some threads I don't know what is dumb and what is smart, for lack of personal knowledge, so I hope other people pitch in those threads) and possibly a signal for flagging. A comment that is BOTH mean and dumb definitely deserves user response to eliminate it. Comments that appear to be spam (what you are specifically asking about) are also against the community culture here, and I would flag those and downvote those when I see them (but I haven't seen a lot of comments like those lately).