There's been this saying floating around roleplaying design circles for a decade or so. It states...
"Say Yes, or roll the dice"
As I've been working through my FUBAR rewrite, I realised that there are some times when it's fine to flat out say "No". But that response needs to be justified.
I've been laying things out with charts, and reflecting on the various times this game has been played.
Most of the time at conventions, a genre is established in the first ten minutes of a three hour session. The table consensus settles into the tropes of "spy thriller", "exploitation flick", "Tarantino", "occult noir", then as those tropes start to bound the narrative, we allow specific elements to push the envelope in order to make a session that subverts those stereotypes in some way. The core tropes set the tone and most players feel comfortable playing within that space, they don't bother trying to do something that breaks the genre. But in several games I've run over the years, there has been a single player who has tried to simply break things... at which point all genre forms are shattered and the game falls into a default mode of "slapstick hi-jinks". When the genre breaks, the game is still fun, but it's no longer the focused genre driven piece that it could have been. You can't put the genie back into the bottle.
That's where I've decided that it's fine to limit the actions in some way, as long as those limitations are backed up by the existing story, and the agreed consensus of the table.
More formally, there is a progression from "Yes" to "No" that should subconsciously flow through the Oracle's mind as they are arbitrating an exploit that may be undertaken by a character within the game.
Yes - The exploit simply happens. If the character has the skills to do the thing, and there's no real risk or massive advantage that could be gained from the thing, and it works to move the story along, just let it happen.
Roll Dice - The exploit could feasibly happen. If the character has the skills to do the thing, but it could benefit from a bit of drama to make the story more interesting, or if the event might possibly cause a noticeable shift in the ongoing narrative, call for a die roll.
Not Quite - The exploit couldn't happen as this time, but might be possible if the situation changes a bit. If the character doesn't have the skills to directly attempt the thing, but it's an event that could feasibly occur within the established oeuvre, then a secondary die roll might be allowed. Such a secondary die roll (if successful), twists the story in such a way that the original aim is now achievable from a new direction.
No - There's no way this exploit could occur. This is a last resort, but if an action is likely to break the developing story it should be used. If a player can justify an action through reference to existing stories/movies/TV-shows/books/radio-serials/computer games matching the setting of the story, where their exploit has been attempted successfully, then a flat out "No" verdict isn't justified, there might be two or three "Not Quites" to push the situation in such a direction that the exploit is now feasible. If the player can't justify things in this way, don't be afraid to use the "No".
With this in mind, I sometimes begin a session by giving players a list of two or three movie/TV-show/book references to get players on the same page with regard to the genre I'd like to see a story generally stick to. Then I get a suggestion from each of the players to add to the list of possible inspirations. Through the course of play a few others generally have the possibility of creeping in, as long as they don't disrupt thing too far.
For example, starting with a Firefly/Dark Matter/Killjoys type of universe, we've already established a sci-fi setting where it only takes a matter of days or weeks to get between planets, where there are pretty much only humans, and where there are dangerous powerful groups controlling space, while the characters exist in the cracks trying to make a living. We also have the potential for AIs, and psychic powers. We haven't completely ruled out the options for alien artifacts (eg. Total Recall), strangely manifesting ghost ships (eg. Event Horizon), or killer alien civilisations (eg. Mass Effect), but we haven't established them from the beginning. These alternate options may arise if the story ends up going in that direction, but we don;t need to worry abut them if it doesn't. On the other hand, none of these settings has eldritch tomes of mystical horror, lightsabers, or blatant aliens living peacefully among the humans, so the Oracle would be safe to say "No" if a character attempted to perform some exploit that manifested such elements into the storyline. If a few players were happy with the suggestion, the response might be upgraded to a "Not Quite", but generally it's safe to say "No" to prevent the story simply going gonzo.