It is difficult, but I have come to the conclusion that in some cases, there is no option but to firmly state that you will not engage in any further discussions on the person's pet topics. Maybe this applies more, the closer you actually are to the person. I'll lay out my reasoning:
- If you engage them in debate, their only takeaway will be that their "ideas" are worth debating.
- If you instead try to be polite and just endure listening to them, this is taken similarly -- they will take it as indicating their ideas are worth being heard.
- If you ignore them with no explanation, this will surely be taken by them as their having won a "victory".
Which leaves only one option: to ignore them, but only after having made a clear and firm statement that this is what you are going to do. Naturally, the "victory" angle will still be in play, but it will not be left as a loose end. And as possibly concerns your own pride, it makes no difference, since you know very well that "victory" would have been declared just the same, in any case.
Beyond this, a phrase recently popped into my head, which I found to be perhaps interesting:
"They trumpet their own ignorance, precisely in order that they might wear the resulting ridicule as a badge of courage."
The context is that I was contemplating how someone could be so blind to reason that it becomes nearly impossible to believe that they aren't actually aware on some level; the questions being, on what level, how, and why? When a person gets in with a group of like-minded people, there exist any number of ways to make an impression on, or raise status within the group. One way (especially with religion-tinged groups) would be in showing how much the person is "suffering" for the cause, and in this case, the more outrageous the claims, the more "suffering" produced, and the more status may be gained within the group.
The mark of such a group might be seen in whether it involves a certain camaraderie based both on is members subscribing to a particular core set of beliefs, and on the "persecution" of its members for their remaining true to those beliefs. For such a group, the more likely to provoke ridicule the beliefs are, the better.
For such a person, your responses will be all they are after, and I'd predict that they'll move to ever more inexplicable extremes to solicit more of them from you. Because it is not actually the debate they are after, but rather only the fact of being opposed; because in a twisted way, to be ridiculed is precisely the thing they are after.
And when this is the case, I think it presents a nearly perfect trap for any who would be inclined to engage them, since it might take someone a good long while to perceive that the person on the other side is not interested in the actual debate, at all.