Let there be a good X where the optimal consumption is 0; i.e the social costs for any unit provided would always be greater than the utility surplus of the market.
We know that prohibiting it( equivalent to an ad valorem tax rate of at least 100%) would lead to maximum incentives for a black market.
As the ad valorem tax rate( or as any amount of taxes do) increases the incentives for a black market increases because the elbow room for profit increases.
One might reduce the consumption directly in the regulated transparent market but they do not know what happens in the unregulated hidden black market. You would just notice the consumption and if it is greater than the regulated market provides you could infer there is a positive supply from the black market or if you prohibit sales in the regulated market any consumption would be one that the black market supplied for.
How could one minimize the total consumption of a good( e.g cigarettes or petroleum/fuel) knowing the fiscalization is less than perfect( bribes, lack of control, etc); i.e the probability of being fined is not 100%(probably much less), and that any single fine should not be greater than the social cost; a principle of fairness I only caused Y damage you can at most fine me for Y?
We also know that when one increases the penalty the cost does not always increase. For example raising the fine might make anyone breaking the law a hero and the social cost of "crime"(buying cigarettes) could decrease.