the genius is John von Neumann

Historical note: According to Prisoner's Dilemma By William Poundstone, von Neumann didn't suggest issuing a nuclear ultimatum but instead advocated a surprise first strike against the Soviet Union. Bertrand Russell did suggest a nuclear ultimatum but with the goal of establishing a world government rather than just non-proliferation.

In a previous related discussion, I noted that I I Rabi and Enrico Fermi did propose using the threat of nuclear attack to deter the development of fusion weapons. However in my online searches, I haven't found any prominent historical figures suggesting the exact thing that you (and I in that previous thread) are suggesting here, of using a nuclear threat just to prevent the proliferation of fission weapons, which is kind of curious...