You can hold some ideas right up until the moment you, or someone else, decide to put them into operation (see KKK, the holocaust, Pol Pot, Jim Jones, etc.)
In the UK, the test for being locked up in a secure institution is “Are you a danger to yourself or others?”
When considering, "Do People Have an Inalienable Right to Believe What They Want?" The upshot of this is therefore, “Yes, as long as nobody gets hurt.”
Unfortunately, it is often not clear when harm starts and, by the time it is clearly seen, the stable door is swinging wide and the horse has bolted.
The question is perhaps clearest when some fundamentalist sect decides that medicine is “of Satan” and only prayer should be used to cure the diabetic child. I feel that, at this stage, all members should be required to sign a document in which they undertake to supply orthodox medical services to all members under 21 and any members who are of low intellect. Failure to do this or failure to comply would result in incarceration and re-education and release only when they have been cured of the delusion.
This may seem harsh but such a belief is criminal negligence waiting for an opportunity to do harm and is no different from wiring your door-knocker to the mains supply and saying, “I’ve rigged it so that someday that switch will turn on and then a random person will be killed.”
Of course, that example is pretty clear; it is the line that is difficult to see.
The other, harsher, view is that people who put out stupid ideas should be banned from publicising them. Fortunately, those who believe that UFOs abduct people are highly unlikely to ever reach a position of real power – so it’s perhaps best to let them get on with it and they define themselves as lunatics and thus restrict their own access to the gene pool.