Two things, firstly having someone defend at zero (I.E. without the benefit of their defense roll) is preventing a defense roll. I don't know how you could be more clear. If you are suggesting that they would still get a roll, but not the benefit of their skill then you're just making rules up (which isn't a bad thing, just isn't something you can stand on in a rules discussion), because nothing else does that (maybe a maneuver/invoke for effect, but that's not enumerated in the rules either).
Secondly if you read the earlier part of the passage (I included it intentionally) it states that a block must prevent an action. A defense is not an action, but is actually a part of the opponent's attack.