It's not very hard to set up captchas. Just have the users fill out a captcha before each post and I think you'll see the amount of spam drop significantly. Also, if possible, it would be cool if users could flag posts as spam. After a certain number of users have flagged a post as spam it could be deleted(but still saved in case an admin decides it was not spam and wishes to restore it). It probably also wouldn't hurt to have another site admin or two.
The level of spam that is present on this website is enough to prevent new users from returning.