Someone who repeatedly crosses the line is not confused about where you drew it.

Repetition removes the possibility of confusion. Once you have said where the line is and it is crossed again, the question is no longer whether they understood but whether your limit is something they intend to work around. That is a different and more useful question. It also changes what a reasonable response looks like.

Stop re-explaining after the second time and start deciding what you will do instead. A limit becomes real through your response rather than through their agreement. That is unfortunate and it is also entirely within your control.