In English, one very common word for a person who refuses to accept the truth of something in defiance of logic and/or evidence is denialist. We can also say that such a person is engaged in denialism.
Merriam-Webster defines denialist as follows:
: a person who denies the existence, truth, or validity of something despite proof or strong evidence that it is real, true, or valid : someone who practices denialism
We often modify denialist and denialism with words or phrases that indicate what the denialist refuses to accept. For example, someone who rejects the overwhelming evidence that climate change is occurring and is caused by human activity is a climate-change denialist. Similarly, those who dispute the scientific consensus concerning the seriousness of COVID-19 or the safety of vaccinations against the disease are sometimes said to be practicing COVID-19 denialism.
Please note that you can use denialist or denialism only when the focus is on the person's refusal to accept something that is true (or that you believe is true; see "one final note," below). You cannot use these terms when the focus is on the person's insistence on believing something that is false, unless you can reframe that insistence in terms of a denial of the truth.
For example, someone who claims that wildfires are the result of attacks by a secret group of Jewish bankers who control a laser that orbits the earth on a satellite may be a conspiracy theorist, an anti-Semite, a liar, an idiot, and/or a lunatic, but we wouldn’t normally refer to her as a lightning denialist, a careless-smoker denialist, or a gender-reveal mishap denialist. Likewise, although we theoretically could call those who dispute the validity of the Copernican model "round-earth denialists," in practice they are generally known as "flat earthers".
So what do you do when the focus is on affirmative belief in something false, rather than on denial of the truth, but you don't know of a term (like "conspiracy theorist" or "flat earther") that fits the specific context? Unfortunately, I don't think there's a single noun that will work in all cases. However, if you think the person is deliberately avoiding coming to terms with the truth, perhaps out of misplaced loyalty to an ideological system, you can accuse him of willful ignorance or self-deception. If you think he actually knows the truth but is only pretending not to, you can accuse him of bad faith. And if none of these quite apply but you want to emphasize what you see as a combination of foolishness and stubbornness, you can use the adjective "pigheaded," which means "stupidly obstinate."
One final note: Denialist and denialism are not neutral terms. I will call someone a denialist if I disagree with him and think he is being unreasonable, irrational, or dishonest. If I agree with him that the evidence for a given position is flimsy or suspect, I am likelier to applaud his well-founded skepticism than to accuse him of denialism. In fact, even if I strenuously disagree with him but do not think his position is unreasonable or dishonest, I will describe him as skeptical rather than calling him a denialist.