I *did* look into the 'Null Vote' option specifically on this occasion. Unfortunately, it doesn't actually let you cast a '0' vote. All it does is get around the annoying radio-box issue of not being able to "uncheck" a radio box.

Instead of having a two element radio box ('-1','+1') there are three ('-1', 'Null', '+1'). The 'Null' option simply means "dont cast a vote on this node," and it comes preselected on all the nodes. So your options are still the same, you just have the ability to uncheck a vote before you send in your votes for the entire page.