I normally use a combination of the two: anonmous user is free to browse everything, but if he wants to vote, then he has to register.
In the registration process, depending on the situation, I use an optin thru mail (to complete registration and confirm that at least the mailbox exists) and/or a CAPTCHA.
From that point on you can decide if the user can vonte more than once, or any other rule.
Btw I'm not a fan of the IP-based constraints: there are a lot of situation in which big organization's network use few IP for all their users, so the risk to block users that could vote is high.
