The best thing you can do is simply vote with your wallet. Don't use them again. 

Of course if we all do this then they no longer fly in NZ and then we go back to the good old days where it costs more to fly CHC - AKL than to Australia.

I would argue that if Jetstar had never entered the domestic market then we would probably still have Pacific Blue which was a much better airline than Jetstar or AirNZ.

I'll fly Air NZ if I really have to, but I would never ever fly Jetstar and I try to avoid flying altogether these days. Admittedly driving from Wellington to Christchurch is at least double the cost of flying, but the customer service on the Interislander is vastly superior to either of the domestic airlines and it's a nice drive down the Kaikoura coast with the added benefit of having the use of my own car when I get to my destination.