They probably won't have to. If Japan goes to war in 1940 the reserve of strategic materials it stock piled OTL in 1941 won't exist. The US simply freezes Japans bank accounts in the the international banks as it did OTL, enact the other embargo. Japan is now cut off from its primary sources of oil, steel, alloys, chemicals, machine tools, half the cargo shipping that serviced Japans ports, ect... Without the reserves built up in the 12-16 months before Dec 1941 Japans industry starts shutting down in roughly 90 days & is collapsing at 120 days, the war fleet runs out of fuel, the winter of 1940-41 sees severe food shortages in Japan, coal shortages, ect.. ect...
However the idea the US is magically blocked from war with Japan is a bit over the top. As I pointed out mobilization is going to start in 1940, the Two Ocean Navy budget would be enacted, 900,000 men were conscripted & plans laid on for drafting 3,000,000 more into the Army. All that was set in motion by the collapse of France, some actions earlier. The US was also fighting a undeclared naval war with Germany in the Atlantic, and occupied 'neutral' Iceland long before any German DoW vs the US. The establishment of the mid Atlantic Neutrality Exclusion Zone and "Shoot on Sight" orders are a useful model for the Pacific. A 200 nm exclusion zone around the Philippines constrains Japanese warships & cargo shipping in the South China Sea. Ignoring that while the US send reinforcements to the PI & weapons, ammunition, & all the other items to the Commonwealth & Netherlands East Indies is kind of stupid for Japan & all to the advantage of the US.