I may be wrong, but I think I remember the old Litestep page or some site that made this crazy thing that each user could only download each theme once. It was the worst idea ever. Completely annoying. Please don't do that. What you could do is make a rating system that each person only gets one vote, and maybe put their names next to it. If someone has enough time and wants to register 20 new names just to get their votes up, then they are just crazy and we should let them have the votes.