Could Nazism have been prevented? If so, by whom and through what actions?

It is unlikely that Nazism could have been prevented, although it may have been contained.

The general thinking is that the economic sanctions put upon Germany as a result of World War I + lingering nationalism from the 1800s + the economic collapse in the late 1920s made conditions ripe.

Without those conditions, Nazis still likely would have existed but may have been a minority party.