The timeline I'm currently working on diminishes the impact of the US on the war by having it wholeheartedly embrace eugenics and racial hygiene - which means a vastly delayed Manhattan Project (no poaching Jewish and Italian scientists from British and European universities), as well as a war economy crippled by race riots. However, I've had to progressively move the P.O.D. back until now it's in the early '20s (with a beefed-up 1924 Immigration Act as a major lynchpin).
Chuichi Nagumo authorising a third wave of attacks against Pearl Harbour targetting the carriers, drydock facilities and fuel stocks is very unlikely given the risk it would have involved, but if the US had provoked Japan into making a more decisive strike, it could have kept America out of the Pacific for a good 18 months. Similarly, no attack on Pearl at all * might have had a similarly crippling effect: it would have kept the US focused on Plan D/Plan Dog (AKA keeping Japan out of the East Pacific), allowing Japan time to absorb its conquests and (correct me if this is incorrect) means the USN wouldn't be forced to rely on the carriers, putting them at a considerable disadvantage against the IJN once they finally decide to get around to Japan (I believe none of the salvaged Pearl Harbour battleships were actually used in battle, as by that point it was obvious they were obsolete compared to Japanese ships). These P.O.Ds, however, still hinge on what someone else does to the US, rather than something the US does to itself, which is probably what you want.
The US Presidential elections 1940 could be another good P.O.D. - I can't see any way for Willkie to win an outright victory (particularly as Roosevelt will always bury him in the electoral college), but another candidate campaigning on a purely isolationist platform could prove popular enough to weaken US response to the war (Willkie promised "unlimited aid" to the UK which meant he wasn't seen as sufficiently different). Alternatively, if the Republicans put up a pro-war candidate (a little ASB in itself), Roosevelt might have to make more hay of his "no foreign wars" promise to win the popular vote, which could encourage him to stick to it (at least as far as Germany is concerned) and avoid deploying troops on European soil.
* Perhaps if the Hull Note made no mention of Japanese-occupied Indochina, which after all had a veneer of legitimacy, and the attack force is recalled.