They defang all of them. They can still be milked because they still produce venom, but if they get bitten, it's by blunt stumps where the fangs used to be and the skin doesn't get broken. It's quite cruel to the snakes actually.
I can't vouch for the king cobra shown here, but before he did this one, he did the same act with a "regular" cobra, then milked its venom in front of us to prove that there was in fact real danger in the act. Photos will be up shortly underhttp://imageevent.com/ralf/thailand