One way that it could have been stopped, or at least greatly reduced in its scope was if countries like England or the United States or France, etc., had allowed Jews to emigrate to their countries. Because anti-Jewish sentiment was rampant within these countries as well, particularly amongst the elite and powerful, very tight restrictions were placed on the numbers of Jews that were to be allowed in each year, thus preventing many Jews from escaping the persecution they faced and the beginnings of the holocaust prior to the start of the war. Even the blockade by the allies prevented food aid and other important supplies from reaching people and helped to exacerbate the situation by increasing numbers of starving, particularly those unlikely to get first priority like the Jews herded into ghettos already, etc.
As the previous poster said, there was very little willingness on the part of other countries to do anything to intercede at the beginning until their entry into the war became politically and economically expedient. No one was going to come to their aid unfortunately until it was far too late.