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.