A prohibitionist, in the sense history books mean, is someone who supported Prohibition: the United States ban on making and selling alcohol, especially from 1920 to 1933. Those people argued that drink ruined homes, so the law should stop it. The word can also name a person who wants a ban more generally, but school English usually points at that movement. It is a label for a camp, not a job title, and it can sound dry or attacking depending on who says it. Though actually, not everyone who disliked drink was a prohibitionist — some wanted less drink, not a nationwide ban.