Joh OK, a save word is a good way but how you prevented they use it not only because they want out of frustration or fulfilling their urge?
A safeword is for when things really go off the rails. When discomfort (whether physical or emotional) has given way trauma. Most of the time it's best to try and forget you have one, and sometimes when things really go off the rails you are too wrapped up in it to realize you should safeword, so it's no substitute for the person in charge being observant and careful.
It's like a guardrail. It may keep you from going off the cliff, but if you need it, you're already getting dented up.
With good communication and caring observation, it shouldn't be necessary very often at all.
It's also important that there be no negative consequences to safewording, and no judgment about whether it was appropriate. If you are afraid to safeword then it becomes useless.
I have a safe word, but I haven't used it in at least ten years. Maybe twenty. I'm not sure. COVID has really messed with my sense of time! lol