Rating or Voting without registration

As "sv3til0" described, an open voting is always a problem. You have no control about the user, as you have by a registered voting or an voting with email activation (send an email to confirm the voting).

Prevent the voting by a cookie is useless. You can restrict it by ip blocking but if different users use the same network router etc.

What you maybe can do, is to allow a maxium amount of votes from the same IP per time (minute, hour, etc) or in complete limit the number of votes.

What you can do with cookies is to set the date and check the database to clean duplicate cookie content (maybe in combination with the user ip).

And you can restrict your voting by using a captcha ...

But the best and only way for clean data is user login.