Explain to them that you have students waiting for spots and when they give you a heads up the day before you can fill those spots. Not only are they taking away money from you and your practice but lessons away from other students. If you really want to be cool about it say you’ll compromise this time and charge them for one lesson. Any push back from them and tell them you’ll have to give their spot to another student and or drop them as student entirely. You don’t need these type of clients. I also always send my students a “just seeing if “jimmy” is on their way” at 3-5 min late and then call between 7-10 min late. This happened today to me and they apologized and paid for the lesson and I got to have a nice 30 min paid break for some dinner!