Being uncivil at times is the only thing you can do. When some says democracy is flawed and we should have a Catholic dictatorship are you suppose to sit there and reason with him? You're wasting your time. Instead, show him how much contempt you have for such an archaic, dangerous and vile idea. It's not likely to change him but at least you'll have honestly conveyed your disgust, so others may be cautious to relate or associate with the "demented fuckwit".
There is another alternative: rather than resort to personal insults, understand that the debate is not about changing the fuckwit's mind, turn to the audience (be it IRL or online), point out his fallacies, and retain your own dignity. While I agree that there are fuckwits in this world, and while in a 1-on-1 scenario my suggestion is irrelevant, to resort to ad homineim is a tacit admission of defeat on the part of the insulter.
When you must, this would be a better tactic. With that said, the faulty assumption is that you even have to talk to or about such people in the first place. If someone is so moved to call another human being that terrible name, deserved or not, it shows that there is a very high level of emotion involved, in which case, it appears the person is simply gratifying their own anger more than anything else. It's far better to simply shrug your shoulders and walk away. When people stand on absolutely blind faith, you aren't going to change their mind. You have zero chance, and all you do in the process is legitimize their position with others. On the other hand, when you resort to self-gratifying insults, you only succeed in making yourself look small and unable to cope with their "arguments."
If you must respond, the best tactic is to simply say, "Look - your position is fundamentally built on blind faith/personal belief system/whatever. There is no way to discuss it rationally because you will allow no facts any contrary implications. You interpret everything to support your view. In the same way a that a word that can mean anything really means nothing, an argument against which there can be no contrary evidence isn't really an argument. It's just something you choose to believe. Now, I know that even this won't change your mind, but I hope it does explain why I think that anyone who tries to have this conversation with you is just wasting their time."
Here, you aren't lowering your own standards with a silly attack. You aren't giving into mere emotionalism as children do. You aren't giving their position any credibility. You are denying it credibility to any potential audience as you disengage. But calling people names -- that's never appropriate.
For the record, this is precisely the position I take with conspiracy theorists of pretty much any kind, be they political (9/11 Truthers, Birthers), religious (Jesus never existed, X is the Antichrist!), or even domestic (I think we can all think of examples of when people have concocted conspiracies to explain the actions of people we know, possibly even ourselves). In my own experience, it's proven to be the best possible approach.
edit: Very good OP, btw. Well worth reading. Again. And then probably again.