If somebody wants to pad the ratings, they're going to do it. I can add whatever blockers I want - restricting by user, IP address, whatever - they're all rather easy to get around if you somebody really wants to. There's no real way to completely lock it down to one vote per person. We do our best to make sure that people don't have duplicate accounts, but it's hard to manage when you have hundreds registering each day. The best thing you can do is report the issue here.