The executive branch could absolutely unilaterally cut off support to Israel. We already have laws that prohibit arms transfers to countries interfering with USAID operations, and we're signatories to treaties that prohibit arms transfers to countries if we reasonably believe they will be used in the commission of war crimes. The easiest one for the president to prove would be the former, since we literally have reports from USAID saying this is happening. It's also worth noting that we have treaties obligating us to provide certain amounts of aid to Israel, but enforcing these laws is the sole responsibility of the executive branch. Biden could choose to cut off arms transfers at any time, and if someone wants to argue that our obligations to provide aid for Israel supersede international treaties they can let the courts sort it out.