This setup is not working as the Random Effector is just randomizing the visibility of the clones... So the clones out side the fall off are not randomized and hence are visible.
This is my solution:
Use a Plain Effector instead of the Random Effector with the same falloff and the visibility box checked.
In the Effector tab of the Plain Effector, Change Max to -100 and Min to 100.
You will see that the clones in within the falloff disappear. Invert the falloff.
Then apply a Random Effector.
Uncheck everything in the Parameter and increase the weight transform to 100.
Create a box fall off just big enough to contain the other falloff.
Invert the falloff.