Could you not identify higher than normal viral scores and run some automatic checks on the links content to look for dodgy behavior (ie executing like / share links?) That would still end up with you being in a cat and mouse game of obscuring dodgyness but its a start.

Have you guys looked into sharing likelihoods of affected users? In other words I rarely share stuff I click on. If a higher than normal number of high view - low sharers like myself are sharing its either extremely popular or its spam. (Worth flagging for a manual check)