Get in contact with a trusted campaign manager. I would recommend you to decide your budget and then contact one manager accordingly. You can take help of the list given here to come to a decision. Afterwards they will set up a campaign for you and you can monitor the traffic to your site in order to evaluate whether its paying off or not.

Overview of Bitcointalk Signature Anti-Spam Campaign Managers
But do note that a better thing to do would be to start taking into account people's complaints than getting new people to join your site.