Quite a lot of historical nations had most civil administration operated by eunuchs, for exactly this reason. Obviously not foolproof, as eunuchs have friends and family, and probably not practical to implement today, of course :)
Plato's Republic[0] specified rules to minimise corruption opportunities. That's what comes to mind anyway.
Excerpt from [0]:
Socrates points out the human tendency to be corrupted by power leads down the road to timocracy, oligarchy, democracy and tyranny. From this, he concludes that ruling should be left to philosophers, who are the most just and therefore least susceptible to corruption. This "good city" is depicted as being governed by philosopher-kings; disinterested persons who rule not for their personal enjoyment but for the good of the city-state (polis). The philosophers have seen the "Forms" and therefore know what is good. They understand the corrupting effect of greed and own no property and receive no salary. They also live in sober communism, eating and sleeping together.
Ok, provision A: in the event of the thermonuclear war, the elected leader should be taken to a protected bunker, where he should be killed in the slowest and most painful way.
You can just imprison their relatives for the duration of the mandate. And them keep them and their relatives imprisoned for another two years after the mandate is over.
There's no need to kill anyone.
Elect only people with no living relatives, and execute them after term's end.
I vaguely remember some fantasy novel I've read many decades ago with this premise... The Goblin tower?