It's a very strong case of confirmation bias. They ALREADY believe a specific set of things, so when something comes along that touches on those things, then all the bits of that that tally with what they think are true, and anything that conflicts with it is not true.
Having a "fixed false belief" in psychiatry is normally classified as a delusion, and a symptom of a delusional disorder. This probably falls somewhere on the spectrum. There are always going to be people who are "lost to reason", and you need to identify them, and probably not waste time with them.
If someone actually has a psychiatric disorder, then you can't sure them by explaining to them over the internet.
NOTE TO CHEMTRAIL BELIEVERS: I'm not saying you are all crazy! Just that some crazy people believe in chemtrails.
Fixed false beliefs can apply to larger, more accepted delusions as well. Such as religions or theories with no proof. thus the method should not rely so much on strict rigidity and adherence to theory, as opposed to applying logic and interpretation to many theories.