Potentially, sure. Things like carpet bombing civilian areas, certain uses of minefields, certain munitions, etc are banned by convention but the thing about the Geneva Conventions are that they are a mutual treaty. Your nation agrees not to do it to other signatories or nations or forces that have otherwise agreed to the limitations as well.
In this case? No. Not at all justified. Hamas might not agree to the Conventions, and thus don’t benefit from the protections, but the moment they suspended elections they lost the right to claim they represent the Gaza Strip, even if Israel didn’t claim it as territory regardless.
Israel is a signatory of the Conventions, and the Gaza Strip is their claimed land, this is a war crime against people they claim as their own, the actions of what amounts to a domestic terrorist network is irrelevant to whether they can starve people into submission.
A standard genocidal tactic of imperialists everywhere, mind you, from the Irish famine, to the Holodomor, to the Bengal Famines.