At the beginning of the relationship, there are (or should be, if you're doing things right) a series of foundational discussions about what each person wants/expects out of the relationship. As part of these foundational discussions, what kinks you consider "hard limits" should be brought up to make sure none of your hard limits are must-haves for the prospective Dom. A hard limit is a hard limit, period. And your partner is obligated to respect that. And by "respect that" I don't mean put their must-have aside; if it's genuinely a hard limit for you and genuinely a must-have for the prospect, you part amicably, no harm, no foul.

Now, ongoing during the relationship, if an aspect of your kink is turning you off, you can have a calm meta talk about that and, hopefully, your Dom will accommodate you and take that off the menu going forward. Again, there is always a chance, of course, that this might be a must-have for them, in which case you have an impasse. If you can't resolve the impasse, the two of you move on.

Now, all that being said, I think you're primarily getting at "What if, in the heat of a scene, I withdraw consent for X and my Dom keeps going?" Well, that's called sexual assault. That's called rape. And the moment it becomes assault is the moment when you use your safeword and your partner ignores it and continues. By using your safeword, you have unconditionally "withdrawn consent" for X, for whatever reason you consider sufficient. And your Dom is obligated to respect that. If they don't, that's rape. That's what happened when James Deen assaulted his partner, Stoya. She said the most horrible moment, and the moment she knew she was being assaulted, was when "he ignored our safeword." You withdraw your consent during a scene by using your safeword. And if that withdrawal of consent is ignored, and your partner continues with X, then it's sexual assault. It's rape. Period. Full stop.