If you really want to disarm a Naked Man with a Samurai Sword and have a bit of fun at the same time, I would suggest pepper spray or even better bear spray. Of course either of these would work better if aimed at the general area at the top of the legs.
I would bet that if the bear spray was used, I probably could have heard the scream all the up here in Colorado Springs.
Of course, the instant he sees the pepper spray can in your hand, he's going to charge you, and open you up from neck to groin. Take a look at this video of police in Nicaragua against a guy with a knife. The fun starts at 1:25.