But if you MUST engage with them, be horrbily, horribly polite. Type in Stephen Fry's voice. They hate that.
He said: "I'm not racist, I just hate ALL muslims"
I pointed out that this was - in fact - very racist, and he might like to cease his all-caps ranting and reconsider his position. After all, this is where proper grown-ups come to talk.
He didn't like my suggestion one little bit, and after a bit of a one-sided to-and-fro, suggested some violent modifications to my body, which to any reasonable person resembled punishments under Shari'ah Law. For example, the removal of one or more limbs so that I would not be able to type reasonable and polite replies in his general direction.
I told him this was a ludicrous suggestion, especially that Siri now comes with voice activation, and that I'd probably work out how to type with my nose sooner or later. I also suggested that while he might be very racist, his limb-removal service in the name of correcting people's behaviour shows that he subconsciously supported the Shari'at of the Muslim faith which he despises.
This was all too much for him, and things, sadly, went rapidly downhill from there.
Remember kids: Politeness costs nothing.