New Zealand Law
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As you probably know, it would be relatively straightforward to get an order under the Harassment Act under New Zealand law but as you also probably realise enforcement in the United States would be difficult. I am surprised your phone service provider cannot block an international call, which I thought was relatively easy to organise. But perhaps you could, if you know her phone provider, contact them and say that you are being harassed in New Zealand which is likely to be an abuse of their phone policy at their end.
The other alternative would be to contact police in her hometown and complain about the harassment. It can be difficult to persuade them to take action internationally.
A third alternative might be to get a US lawyer to write to her and threatened to sue in the United States. That may be affected because she is aware of the difficulties of dealing with these issues across borders, but if she gets a US lawyer threatening to sue her that may be more effective.