We can leave behind radioactive material, and take only that which is not. This of course leaves the 3rd Density world left behind a rather unpleasant place, but the 3rd Density world is not intended to be a world that supports life. We can also simply alter radioactivity. Man tends to think of this as something that takes thousands, perhaps millions of years in half life spans to affect radioactivity, but what causes radioactivity is the structure of the atom and the nucleus, and this can be altered by chemistry just as the nature of hydrogen and oxygen forming water is an alteration. All atoms are affected by what they come in contact with.
None stand alone.
All of them are affected. This is why we have made the statement that man would be allowed to destroy his world in a nuclear war, but likewise we can interfere with other steps that are being taken and can influence the state of the world. Basically, we can't interfere with the lesson, and where the lesson won't be interfered with we have the right to do so as future residents.