Irritating letters

I have received two letters from Direct Axis and I do not recall ever providing them with my physical address. I do not know where they received my address from but I am highly irritated with the letters that I am receiving. I attempted to phone the call centre, not once, not twice but three times to stop the letters from being sent (as stipulated on the form) and no option was provided to speak to a consultant. The letters to me are to stop immediately. These unsolicited, annoying letters must stop being sent.