Maybe have a special tag that you can add to the text for info that you only want karma users of a certain level or higher to be able to see -- ex. (karma>300)[Contact me at my@email.com], or instead of direct numbers, target people that are able to downvote, or have a moderate level of karma. People would post a link to the recruitment page/job description page on their website for all other users, which would hopefully work at deterring spammers from contacting them personally.

I think something like this would help you focus your recruitment efforts on those who have at least contributed to the community in some way, which should filter out people spamming every single email in the thread.

Another idea is to mask emails with a craigslist-like mailing address, which would give the end-user the ability to report an email as spam, and therefore tie that email to the offending party's hacker news account.

Edit: What I mean is that each hacker news account would see the email address as a different one, so when they emailed that account it uniquely identifies the account that originally viewed that email address. So, Spammer A sees Poster B's email address as hn-49384932842@ycombinator.com, and Legitimate Candidate C sees Poster B's address as hn-4494838943842@ycombinator.com. When either one emails that address, if Poster B reports the email as spam, and if enough reports accumulate, the HN account sending the spam can be docked karma and lower them below the threshold allowed to view further posts.