How did john snow prevent the spread of cholera?

He did not. The accepted view is that Dr. John Snow played an important role in tracking the source of a major cholera outbreak in London during 1854, through his creation of a spot map of cholera cases. His work contributed to the understanding that cholera was spread through contaminated water sources rather than through airborne transmission. However, it was not Snow himself who prevented the spread of the outbreak, but rather the actions of the local water board, which removed the contaminated water pump handle.