Liquor Prohibition is a general decision available to all civilized nations. It requires that the nation has researched Social Science. On top of that it requires that the event Temperance League has happened, and that the player has taken the option An excellent Initiative!.
There is also a list of political conditions, where at least one of them has to be present:
- at least 30% of the POPs think moralism is important
- at least 60% of the POPs are liberal, or
- the goventment type is a dictatorship, either Proletarian Dictatorship, Fascist Dictatorship, Bourgeois Dictatorship or Presidential Dictatorship. Monarchies doesn't count.
Unlike what would be logical 
liquor and 
wine are still consumed by the population. Furthermore it can still be produced in the country, and Liquor distilleries and wineries can still be build.
It decreases tax efficiency by 5% and gives 0.05 militancy pr. month for the entire population. The effect lasts until the rest of the game, or until it is repealed with the decision Repeal Prohibition
All in all the decision actively hurts the country and is one of the worst decisions in the entire game. The AI will always taken the decision, if the requirements of moralism or liberals are fulfilled, but not if it is only possible due to being a dictatorship.