Hummm, I get your point, it happened to me in a Allied game.
I had to cancel the convoys, yes, the USSR is not supplied, but I took a very aggresive stance with UK and US using that MMP so the Axis had to divert resources to counter the Allies ( If I remember correctly, I invaded Portugal and entered Spain through there )
In any case, when the US enters de War, another gamey tactic is just defend Hawaii, and move the whole US fleet to the Atlantic. Combined with the Royal Navy, the German Fleet should be no problem, even in that narrow area.
And because its there, it cannot escape either.