So’s a gun
Good, so we agree. 
We do when those stupid mistakes entail the use of force against others, whether or not that use is voluntary.
Again we are in agreement. As soon as force enters the equation then stopping that use of force becomes the job of government.
Here’s an assertion:
If a drug, used a single time, necessarily created an inability to act rationally wrt to others’ rights, it would be necessary to ban that substance.
Please lets try to keep this conversation in the realm of reality. I’m not talking about a magic evil drug that makes people violate rights I’m talking about the real world where drugs can be used for years without the user ever violating someone else’s rights.
From that moral principle (assuming you agree with the assertion), it is a matter of degrees to decide if the use of certain substances, and the consequential inability to behave rationally, should be banned.
You can not draw a moral principal out of a fantastical hypothetical.
If you disagree with that, then please provide a solution for the problem of permanently morally-disabled drug addicts.
I don’t have to provide a solution, the law is already in place. You violate a persons rights and get caught you get punished.
Does that solution entail jails and asylums?
Absolutely, the difference being that unlike today the jails will not be filled with a population who’s only crime was to break the governments prohibition on drugs (80% if I recall the statistic correctly) and those adicts might actually get some real help for their problem.
– who will pay for them?
Anyone who decides to contribute to them. I believe the justice system is a legitimate function of government is it not? Now the Jails would probably be privately owned and run, but that is another issue.
Does the solution involve killing the addicts? Who will fire the bullets, bury the bodies, pay for the process?
Where did this piece of nonsense come from? B)
If an action is intrinsically (yes, intrinsically) immoral, that is, if it leads to involuntary behavior that impacts (with force) the lives of others, then government has the right and responsibility to restrict that action.
What am I missing?
You’re missing the fact that drugs do not ALWAYS lead to force. There is no law of the universe that says it is and must be so!
Your also making the government the arbiter of morality. Morality has nothing to do with government, or more correctly the government does not set the bar for what we are to consider to be moral. That is the kind of thinking that brought us such great ideas as prohibition (on alcohol), sodomy between consenting adults as a crime, the war on drugs… the list goes on and on.
Government aught to be concerned with one thing… protecting individual rights. Not from possible action but from actual action.
Your comment on guns at the beginning of your post is quite telling. The old saying is true you know. Guns don’t kill people, people kill people.
To make that saying applicable you might say Drugs don’t kill people, people kill themselves… and therein lies the truth of this matter. People kill themselves in all sorts of ways every day are you going to outlaw alcohol, gambling, fatty foods, teen drivers, riding a bicycle without a helmet?