The nids do eat everything and leave it a barren rock. However leviathan headed out into ork space and it was a square stand off. the big problem being that open conflict increases the ork proccess of growth. When at war the orks grow bigger, putting on more and more muscle, and not to mention brain power. Conflict grows orks. just like conflict is the best way for nids to expand, orks increase size, number, ferocity in conflict. They live for war and fighting. It is the only thing they are made for.
Also the ork genetics is so ruggid and their immunr system so high im not sure a virus would work. Orks do not seem to suffer from wound infection or disease at all. Whole limbs can be transplanted onto them with only and i codex quote '0.0023% chance of tissue rejection' orks can not be wiped out and let there remain any habitable planets in the galaxy