If you have to repeat yourself more than twice, then take away the offending item. They will learn the lesson pretty quick believe me. About a year ago, I had an incident with my then 13 years old daughter, I embarrassed her so bad that now she pays attention to what and when I say something. My daughter had the bad habit of calling me to go get her from school/friends/movies, etc. and when I get there still had to wait 15 to 20 minutes for her to come out. Well, so happens that the day of the incident, she had an appoint to get an MRI, they had given me the appointment at 8:15 pm and she had called me asked me if she could wait for me at her friend's house, which I said yes to. Told her that I would pick her up at 7:30 for the appointment and she said OK. Well before I left my house I called her, she did not answer, I did not know which was her friend's house, I knew the street, so I am trying to find out the house number and she is not answering. Called my oldest daughter, who knew where the friend lives and she tells me, I don't know the house number but is the one with the red door, well there are 5 houses with a red door (this are townhouses). 30 minutes later no luck finding the house, I knock on 2 red doors and of course they were the wrong ones. My hubby and I kept calling and texting her phone without any luck.
Finally, I made my oldest come to the neighborhood (she was about 5 minutes away) and showed me the house. When I knocked I was so mad that the first thing I did was start yelling in front of the other parent, my daughter was upset told me that I embarrassed her in front of her friend's mom. Then had the chick to tell me that she did not answer because she did not want to be rude and have the phone at the dinner table. Uh..she would not do that at my house, the phone is a permanent attachment of her hands. So I told her that she did not want me to blow off again in front of friends and parents, she better listen to me and keep track of the time, that her phone is for me to be able to reached her whenever and not for her to be with her friends all day on it.
She has never made me wait again and better yet answers the phone at the second ring when I call, problem solved.
Take the device away if and when they don't listen to you and see if they will not like it, they will be more attentive to your words next time. if they did not watch their hair on time and went to school looking like witches then it is their own fault.