I think the most effective short term way would to to provide a tertiary recruiter type account and have users be able to opt in or out of messages. Some people who are actively seeking work would be able to get the content they were after on your site and the recruiters would be able to pick from applicants who most importantly wanted to receive their messages.

For recruiters who take the method of making a normal account and using it to recruit to people who have opted out you could have a report ticket based system, even algorithmic-ally limiting the account abilities of people who repeatedly break the rules should they have a certain comment/report ratio.