Don't you think it's more likely that the Spanish flu was so widespread because of the unhygienic conditions and stressed infrastructure caused by World War 1?
If you're asking about hand washing practices in that era and if that would have prevented the spread, absolutely. Hand washing attenuated SARS spread during the 2002 epidemic by roughly ~50% [1]. I am not an expert in history so I cannot speak to hand hygeine practices during that era. What current research is suggesting is that H1N1 spread was exacerbated by a lack of knowledge to the public [2]. Time and time again public health is predicated on the right knowledge given as quickly as possible; this is the most important key to almost every single disease spread[3].
1)https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3323085/