A lot of spam/spammy submissions are submitted by new users, users that create an account just to submit these links. I guess a lot of them do it to generate google juice since google indexes the site in minutes.
In addition, the number of relevant submissions are not see on the new page thereby reducing the quality of the content on the forum.
Can you introduce a gestation period for new users -- this could be based on number of days since they created their registration or the number of comments or their karma. Once they pass this threshold, these users can submit stories.
BTW, they don't get any Google juice, because links are all nofollow unless they have a certain number of points.