Defensive monster moves

This block thing is very useful. The way I've been doing this lately is this: The player makes a move on an enemy and rolls a success. But I've decided that for example this bodyguard he tries to disarm is the very best fighter in the city, so I deny the player the result of his move. The player gets an XP instead, and a clear description of why his move did not work. I don't tell the players up front that such and such a move won't work, only if they stumble upon it by trying. Clues, of course are abundant, but no discussion whether a move will work beforehand.

For us it works like a charm. Actually I think this is the very best way for marking XP: you encounter something new.

There's something I don't like about saying no but giving out XP.  I think you can flat out say no if you can point to a narrative reasoning that makes sense.  I mean, I get the "encounter new things" but that's a whole different situation - that's a custom move for your whole game, isn't it?  "When you encounter a new thing, take 1 xp" which is cool and neat if you want fast expansion and focus on wide-eyed learning of NEW AND COOL STUFF but it's a different beast than the sort of "sorry I fucked your move here's XP" thing. 

A roll made is sacred.  Dice hitting the table are ineffable, in their own way.  You can say "no, that's not a move, you don't get to roll" (like, you know, the player says "I rush at the master duelist, swinging my axe and screaming" and you're like "okay, cool, but he just bats your blows aside, he's a master, obviously.") but I wouldn't take away results and give out XP in that fashion.

Yeah, I'm with Adam. I think what you mean is "you haven't triggered the move yet," not "your roll didn't do anything."

Actually I haven't ever done it by not rolling at all. Linking XP to novelty with this "roll but fail" method and avoiding arbitrary DM decision on handing XP out was a good thing. On the other hand there certainly was some awkwardness when I nullified the move and also responded with a hard move instead of a soft one. I'll experiment some.