Hmmm, first of all my friends never say stuff like that and the other people who say stuff like that are usually not really worth talking to anyway.
It goes something like this:
PC user: Macs are rubbish, why use them, PCs are better.
Me: Oh, ok. If you say so. or:
Me: Whatever. or:
Me: You're correct, I just do this to punish myself, happy?
Seriously, it's not really worth arguing. Why do so with oviously narrow-minded people. That's like arguing with people that say the world is flat, would you do that?