No, we accept road traffic accidents as fact and do not shut down entire road intersections due to deaths that occur. The same thing applies to child pornography, and 0.5ppm is certainly a tolerable amount in an extremely large dataset.

There are something like ~42000 traffic fatalities in the United States every year, we do not adopt draconian measures in dealing with it, even though loss of life and severe life-altering injuries occur. And such traffic accidents are absolutely devastating for the victims and their families.

Unless the dataset is particularly severely contaminated, the known illegal images should be removed and the rest of it should be allowed to stay up.