Nissan smoke

WE FOUND our new Nissan Navara STX diesel was blowing black smoke from the exhaust after its first service at 1000km. We took it to the dealer and were told to run it in some more, but it was still blowing black smoke after another 500km. The smoke was so bad other motorists were stopping us to tell us about it. After weeks of testing Nissan instructed the dealer to change the injectors, but that didn't fix it. After more calls to Nissan and waiting weeks we still have no answer. What should we do? We're afraid the EPA will fine us.