how can proofofbrain get rid of cheaters in the community
I agree with @scholaris.pob that community policing is far more effective than relying on a small group to do it.
With that said, I personally believe the 'mute' function is far more powerful than downvotes. Although the 'mute' has to come from the token's issuing account (in the case of POB, that is @proofofbrainio), that functionality can be delegated (via the issuing account granting posting authority to those tasked with monitoring and managing the 'mute' function).
Another option is to offer a 'pre-screening' feature to curators, wherein posts are pre-screened for plagiarism. Some of my students and I are experimenting with this approach, with the Proof of Blind project, which you can read about here, here, here, and here.