I have to disagree on you on this specific case. As I see it, the OP has a website full of images and he wants to give the visitors the abillity to specify how much (or not) they like the specific pictures. Possibly to create a "best of" list.

Shutting down people may result in the loss of a view votes (either positive or negative votes), which is bad. Allowing users to vote as much as they like may result in heavilly influenced votes, for someone could vote a hundred times on "5 stars", just to see that picture rank highest. Blocking on IP for a certain ammount of time may stop those abusive users to a certain degree. You must be quite determined to reconnect your connection a lot of times to get a different dynamic IP address, or find a bunch of proxies, which takes time and effort too.

By forcing users to register solely to vote, you probably lose votes too, for a lot of people won't take the hassle to register -IMHO-, and skip the voting, they otherwise might have done.

Like I said, it's choosing between evils and our opinions seem to differ on this point. It also depends on the ammount of traffic, as I stated before.