"stop sending me how did we do messages"

I recently got a how did we do message after having an appointment with one of your staff. I find your continual sending of these messages to me a very offensive invasion of my privacy. Two weeks ago I got one after another appointment where it says "if you do not wish to receive further texts from the trust please reply STOP". I immediately replied with STOP and sat back happily knowing I would never hear from you again. This week, after my next appointment I was appalled to find you didn't bother taking notice of my wishes and sent me yet another unwanted text. This time the STOP message continually came back as "not delivered" - all 36 times I sent the massage (you are legally obliged to give an opt out method). I gave my personal mobile number so I could be informed about upcoming appointments or test results etc not to be an uninvited guinea pig in your research. Stop sending me unwanted how did we do messages or I will have to remove my number from my records putting me in a possible position of harm