这句用了很多隐喻，sterilized与poison less 相响应，暗指authority与power都是种双刃剑，制度与教育可以使其保留好的一面，即为群体服务并且只求荣誉。

So the authority of the guardians will be sterilized, and their power made poison less; their sole reward will be honor and the sense of service to the group.
因此，护卫者的权威将被“净化”，其权力的毒性得以削弱；他们仅有的回报，是荣誉以及为群体服务的意识。

细节解析

1、sterilized：to clean (something) by destroying germs or bacteria消毒；to make someone or something unable to produce children, young animals绝育
Nearly 300,000 men and women in Peru were sterilized as part of a government-run birth control grogram in the 1990s.
在文中，比喻去除护卫者权威中可能存在的自私、腐败、滥用。

所在段落

The Story of Philosophy《哲学的故事》第1章Plato第7节VI. The Political Solution第6段第6句：

This arrangement will make it unprofitable, as well as dangerous, for the guardians to rule as a clique seeking the good of their class than that of the community as a whole. For they will be protected from want; the necessities and modest luxuries of a noble life will be theirs in regular provision, without the searing and wrinkling care of economic worry. But by the same token they will be precluded from cupidity and sordid ambitions; they will always have just so much of the world's goods, and no more; they will be like physicians establishing, and themselves accepting, a dietary for a nation. They will eat together, like consecrated men; they will sleep together, like soldiers sworn to simplicity. "Friends should have all things in common," as Pythagoras used to say (Laws 807). So the authority of the guardians will be sterilized, and their power made poison less; their sole reward will be honor and the sense of service to the group. And they will be such men as from the beginning have deliberately connected to so materially limited a career; and such men as at the end of their stern training will have learned to value the high repute of the statesman above the crass emoluments of the office-seeking politicians or the "economic man." At their coming the battles of party politics will be no more.
浙江大学译本： 因此，护卫官的权威经过了消毒，他们的权力是无害的；他们收获的唯一嘉奖便是荣誉和为人民服务的责任感。

上句见：Day463