I don't see why not. There might be drugs the use of which is not inherently immoral but which it would be expedient and just to ban. Or the possession of certain types of firearms could be banned, though that is not inherently immoral either. Or in a certain situation, it might be necessary to institute a curfew and prohibit citizens from being outside after a certain hour.

These may not be thought as sufficiently "significant" limitations on people's freedom to answer the question. But then how significant are we talking? Greater restrictions on freedom should weigh more heavily on those in authority, and the standards for restricting freedom will be correspondingly harder. But it is hard to see why any increase in mere magnitude will ever result in a new kind of moral requirement: that no prohibition of the act in question is allowed.

Dry and Uninspired wrote:

So at that point, it basically comes down to utilitarian considerations, weighing the pros and cons and the like?

Yes. Or, at least, utlitarian in the broad sense of concerned with consequences, accepting certain inconveniences in order to achieve other goals. I don't mean that there is a utilitarian calculus, or that the production of a large quantity of good is the sole ethical principle, or that good is quantifiable at all.

Dry and Uninspired wrote:

On a serious note, how exactly do we go about determining which of these things should be prohibited? We have potentially harmful activity x which is currently legal, and we have a certain amount of x-related deaths and serious damage to health. Let’s say, for the sake of the argument, that we know for certain that passing a law that bans activity x will reduce the number of x-related deaths over the next 10 years by a certain percentage (after taking into account every single factor, including externalities). How big should that percentage be?

I don't think that there is any general answer to this question.

Remaining very much in the abstract, the answer, if there were an answer, would depend on whether the activity was itself valuable. Compare going outside after 8 pm with smoking. It may be that in a certain city, by instituting a strict curfew one could curtail murders and thefts by p%. But maybe that just isn't enough to tolerate that encroachment on people's liberty; it prevents people from doing all sorts of things, like visiting friends. But maybe one could also cut down on smoking-related deaths by p% by banning smoking; if smoking is not a valuable activity, then perhaps that is fine.

And then there are, of course, various questions about costs of enforcement and other consequences. If you tried to ban smoking, would a black market crop up? Would more people try other more dangerous drugs? (Probably not, but I'm just floating hypotheticals.) Any lawmaker attempting to pass such legislation also has to ask: how will people respond? Say one party tends to make good policy. It may be that the restriction on liberty involved in banning smoking is "worth it," but any party which does so will be destroyed in elections, in this case putting the other party which makes bad policy in power. We find then that a much more prudent way for reducing smoking-related deaths is giving people the option, once they are 18, while restricting smoking in other ways (bans on advertising, warning labels on cigarette packs, etc.).

I suppose that I think that ethics isn't primarily about large-scale planning at all. Sometimes there will be issues that require legislators' attention, and then they just have to do something. In lots of cases, both allowing and prohibiting an activity that has some negative consequences are reasonable options. It may be that the sort of statistics to which people point when they are trying to justify their policy preferences lend almost no evidential support to their views. (I think this is the case on topics like the minimum wage. At some point--it may be at a rather high point; that is an empirical question--increases to minimum wage are offset by increases in unemployment. What sort of tradeoff is acceptable? Almost no one who comments on the topic answers that question, though it would seem to be a precondition to a consideration of empirical evidence. If you read many authors on the topic, you will find them simply citing statistics as though their evidential significance were obvious.)