The delusional thinking apparent in a person is only the tip of the iceberg: the structures go way below the surface and form the individual's identity and worldview. In my case, this was deconstructed from the inside, because I was willing. People can and did point out I was away with the fairies, but did not force the issue, as I needed something else to fall back on first. My identity and my whole 'personal universe' cannot be taken away and replaced with a new alien one, at least not by force and not without willingness. I had to be sure I could rely on the new system that was being built before I was willing to give up the old one. What brought about that willingness? Absolute desperation from being stuck in an untenable situation with no way out but complete abandonment. Until then, people left me be.
If someone believes their teddy bear protects them from ghouls under the bed, don't take away the teddy bear and don't force them to see there are no ghouls, because you can't: if you lift the counterpane, just because you can't see ghouls does not mean they can't. They can and will indeed construe the shadows to be ghouls.
Instead: time will do the job you can't.