Nissan smoke problem

I bought my D22 Nissan Navara STR in 2010. It was six months old and had done less than 50,000 km. I immediately noticed a smoke problem. At 2800 rpm or more it produced large amounts of black smoke until a constant speed was reached. Nissan had the dealer look at the problem and after being called in no less than a dozen times have concluded that the smoke is not smoke but soot and they can do no more, surely soot is a problem in itself and would exceed EPA guidelines. Nissan say via the dealer, they have done all they can. What should I do now?