What early actions could have been taken by the Allies to stop Hitler's actions against the Jewish population?

An earlier action could have been taken by capturing Hitler off guard and sending him to a Internment camp.

Answer

1. Participants of the Evian conference of 1938 could have provided asylum to German Jews.

1. Britain and France should not have declared war on Germany under the pretext of saving Poland - neither they meant to protect Poland nor did they, never decalring war on the other invader, the USSR, and accepting a post-war settlement under which the USSR kept the annexed part of Poland - in fact, parts of the ex-USSR, namely, Ukraine and Belarus, are still keeping those parts of Poland to themselves.

2. Roosevelt and Churchill could have abstained from the demand for "unconditional surrender" of Germany at the Casablanca Conference in February 1943 thus clearly prolonging the war and suffering of the European Jews.

3. Jewish people could have been educated to the fact that guerrila warfare was illegal under international law and thus constituted a war crime in itself.