I personally would write them off, I know it's easier said than done if your bookings are feeling sparse but time is money.
I had a lady that kept 'no-showing' me and coming up with all excuses under the sun when she finally contacted me. In the end when she tried to re-book I sent her this message. (Which I've kept for the future).
Hello .....,
Thank you for your loyal custom, unfortunately due to 3 missed appointments I am unable to give you priority booking. Please feel free to contact me on the day you wish for an appointment and if I have a space I will happily book you in. Thank you for your understanding.
This really worked for me (This time anyways), I also don't think I'm being rude as she was much ruder contacting me after the planned appointments . I find honesty is the best policy. My friend told a client she had stopped mobile and the client found out!!