1. maintain a list of controversial sites
2. every time someone attempts to submit a link from that list, s/he'll get a warning message stating that should the moderators find this submission lacking, 5 karma pts will be docked. No appeals will be allowed. This is the risk of submitting from a blacklisted site. And only users above some karma threshold can submit from the blacklisted sites.
This would still allow possibly useful information from those sites, but also punish irrelevant submissions.