Or simply allow HN members to flag blatant recruiter postings, and after 3-4 flags (or whatever) the post gets removed and the person's account gets blacklisted

Or simply allow HN members to flag blatant recruiter postings, and after 3-4 flags (or whatever) the post gets removed and the person's account gets blacklisted/deleted/warning sent depending on the number of offenses. It would be easy to tell a recruiter's account as the account would be fairly new, with few posts and low karma.