I often wonder how much of what’s going on in the high school scene, college scene, and beyond is a symptom of how we allow “no” to be ignored early on.
“Boys will be boys”, or “It’s not that big of a deal”, or “He was just playing.”
No means No.
Stop means stop.
If a 3yo asks you to stop chasing them, stop tickling them, stop. And don’t use those words yourself unless you mean them.