Ask HN: How does Threadless prevent frauds?

Does anyone know how Threadless prevents frauds from their voting system? I posted this question earlier on stackoverflow, but somehow it got deleted.

For Threadless, users are part of their business model. Threadless prints shirts based on its users' votes, at least from my understanding, so they can eliminate any unnecessary costs on producing products that few people demand. That makes the voting system one of the most important features on the site. And if that's the case, wouldn't bots/frauds be a serious concern to Threadless's voting system? I mean, the only way I can think of to prevent people from voting designs multiple times are either using IP log or cookies (or both). But I've already seen bots that bypass these restrictions on some other sites. So how does Threadless actually prevent bots/fraud which can potentially bring the company down?