Bella spreads her hands. "I don't have a way to put them on a comfy space prison colony. If I were handed a lot of Yeerks in slug form who wanted to be Yeerk Switzerland and sit out the conflict, I could sit them on the blue box and then give them all pigeons and supervise them for two hours to make sure they could wreak only limited havoc if they changed their minds, but I have no way to be sure we can trust any given Yeerk with even the powers of flight and thoughtspeech, given what they as a group or a political unit or whatever are doing with the powers they currently have. I don't have a way to render them magically incapable of infesting people. I might be able to kill them, and as long as that's all I have, this planet needs to be as uninhabitable-to-Yeerks as I can make it until the Andalites come along to save the day, if ever they do."