Let me know when you are ready to stop.
When my child, or other children I have worked with start having a tantrum or start displaying an unwanted behavior that they are unwilling to stop after being asked, I do the following: I look at the child and calmly say, "I am going to step away and not listen because you are not doing what you need to do. Let me know when you are ready to stop." Typically, the child I do this with stops their behavior within a few seconds and says, "Ok, I'm ready now." Then I go back and talk with them about why they were doing the unwanted behavior and what they could do differently the next time to express themselves to me in a more appropriate way.