I saw this posted by a friend on Facebook, and I just loved it! As I have said before, you ought to know exactly whom you are messing with before you start.
This video is pretty instructive. You can see the clerk assess the situation, grab the stool, block the machete, then close with the dude to keep him from swinging. Then he wrestles the machete away and turns the tables.
I was pondering the use of a machete as a weapon, and decided that it is a pretty crappy weapon. It is really designed for bushwhacking. It is really heavy so you can cut down branches, and it is designed for slashing, not thrusting. I would be more afraid of a knife than a machete, because it can be thrust, and because it would move much faster. A machete could cause serious damage, but is too unwieldy to really be effective.