If I see something like that, I flag it. If both the question and the answer come from accounts with no history of stories or comments, it's almost certainly spam. (They're not always newly-created accounts. Some spammers seem to let their accounts sit around for a while before using them.)

Fortunately, I haven't seen too much of this on HN yet.

(To flag a comment, click on its time stamp [e.g., "10 minutes ago"] and you'll see a "flag" link at the top, next to the "parent" link. If enough people flag a comment, it will get killed.)