Suboptimal workmanship

Following a quote by Mopani generators, I provided parts for them to overhaul my generator. It ran for 1 day and the next day the sludge in the oil returned. The gents failed to fix it. They are fixating on water and oil mix and ignoring water pump, heat oxidation, diesel or soot mix, oil degradation and proper drainage. They now demand full pay for a generator not working plus my parts wasted. How do I deal with such below par workmanship?. My generator is worse than prior to their intervention.