Framer is right.
Every ICAO-compliant operations manual ever written will say that the captain has ultimate authority, and indeed he does. Particularly when the fertilizer hits the N1 rotor. But in certain dictatorships and half-arsed democracies, if some management wallah wants minimum fuel for maximum payload, or has published a schedule that stretches the jet's range, a pilot wishing to over ride this would soon be unemployed at the very least.
Said pilot may get away with one diversion with no more than tea and bickies and maybe a suspension, but, knowing this, he would not want to be the only one on that day who diverted. So, then it's a case of who will break ranks first. Who is the pussy? Whose balls are big enough to defy management?
ICAO does not assist pilots in jail, or run an employment agency.
In the bad old days, ATC would often close an aerodrome. They can still do this, but rarely do. Whether they should is debatable, but it would certainly force those who turn up on our shores with minimum fuel to divert while still able to. Or declare a fuel emergency. If a pattern emerged with a particular operator, they could then be shut out of our skies.