We check to make certain people aren't ranking formulaically, and if they are we invalidate their votes and ban them automatically. We also check how close people's rankings are to the consensus retroactively and do some fancy anti cheating patterns there as well.
Would you mind to tell a bit more about the exact logic and algorithms you use for those checks? I'm working on project in a totally different space (offers) but it shares the same idea where users vote on other users submissions and get credits for doing so (to incentivize participation and keep it fair). I haven't really worked out how to prevent abuse, so I'd be curious to learn from others. If you can share things (even just resources) I'd be super happy. Feel free to shoot me a mail to ca-rankpicstuff@willscher.com in case you don't want to make things public.