Do you allow links in your posts? If not, there go 50% of the spammers.<p>Does any money change hands? What niche is the site in?<p>Whether or not you choose to have a pay wall based on limiting spam, most people may see it as a greedy attempt to generate extra money.<p>If it's feasible, create a simple moderation que. Also consider a simple "report spam" button on all submissions.