As both a Requester and a Turker, I was torn on posting this, but we have to punish those who take advantage of the idea that categorization HITs can't be rejected.
Requesters: if you have a REALLY BAD SCAMMER doing your HITs, and it's been less than an hour since they did them, go to your batch in the GUI and click "Manage HITs Individually". Find the HIT the scammer did and click "Review Results". You can approve or reject there.