Is It OK?

[icefractal]

I think the OP needs to be more clear what's meant by "nullify player choices". Because I can think of at least five meanings that could apply, and the answer is different depending which one (and the circumstances, obviously).

1) Action Prohibition
Player: I stab the king!
GM: No you don't, your character doesn't do that.

2) Hard Negation
GM: The BBEG casts Teleport and he's gone.
Players: While under Dimensional Anchor, and standing next to a guy with Mage Slayer?
GM: Yeah, he just does. Power of plot!

3) Soft Negation
Player: I use Detect Evil
GM: (knowing that the BBEG doesn't actually have a way to block this) Um ... no evil detected.
GM: *makes mental note to later claim the BBEG always wears anti-divination charms*

4) Hard Meta Negation
Player: I'm going to make a really stealthy spy type.
GM: Ok.
GM: *makes anything important be guarded by un-bluffable guards with special senses that can't be snuck past*

5) Soft Meta Negation
GM: ... and so you'll need to seek out the reclusive sage Zagyg and convince him to translate this.
Player: Oh, no need, I actually know Ancient Thassilonian
GM: Huh ... well, you read it and it ... partially describes where the portal is, but it has some references to the 'Cycle of Stars' that the directions don't make sense without. You'll need to seek out the other reclusive sage ... Zyzag ... and convince him to show you the scrolls.


I made it obvious for the sake of example, but Soft Meta Negation can be pretty subtle - imagine that the GM never got as far as mentioning Zagyg, for instance. Another example would be something like:
Player: I hack into BastardCorp's database and see what they're up to.
GM: There's a warehouse near the docks that's not officially on the books but has a lot of expensive equipment being sent there, and a security contract with some heavy duty mercenaries.

Seems legit, right? But secretly, if the player hadn't hacked the database:
GM: One of your contacts sends you a message ... "There's something suspicious going on at this warehouse near the docks. Suddenly it's got heavy duty security and they're moving equipment in during the middle of the night. I saw a BastardCorp logo on one of the boxes, but it looked like it was supposed to be hidden."

So in fact hacking does nothing, it just gives you information that the GM was going to provide anyway. But it's entirely possible that the whole campaign happens without the players ever realizing that choice was false.

Soft Meta Negation can even be in the PCs' favor - for example, a situation where "Any reasonable plan by the PCs will succeed" or "Whoever the PCs decide is the murderer, is, as long as it makes sense". Many people wouldn't mind that, but it's still making the choice fake, IMO.