The X Card
The X Card was created by John Stavropoulos and is explained in this document.
To use the X Card, display a hand sign, an ASL X, to indicate that something someone just added to the game is unacceptable to you. It could be a particular topic, event, or even just a word. It might be something that bothers you viscerally or just something you think disrupts the scene. This is an exception to the “Don’t Contradict” rule.
If the person understands what you mean, they should immediately revise what they added to remove the unacceptable thing. They shouldn’t ask for explanation or justification or check in to make sure you’re all right unless you ask for help. If it’s unclear, they can ask for clarification, but only to identify the problem. If you feel it necessary to learn more, do so after the scene is done.
The X Card doesn’t replace your normal ability to talk about something, ask for a break, or excuse yourself from a scene. It’s just a fast way to give a hard no and move on. You should feel good about using it as often as you’re moved to.