The answer is you can't. They did not reason themselves into their position, and you can't reason them out of it. It's irrational to have rational discussion with the irrational.

This is why derision is a valid philosophical debate strategy if a person refuses to engage in rational discussion. To label GP as an R-word is a social signal and warning to others not to behave similarly. Derision also promotes healthy skepticism of new arguments you hear, lest risk falling into the same trap and have oneself become a laughingstock.