Irritating SMS's about alleged 18 year old debt

I have received a number of SMS's from this business. I am with Clientele Legal. MBD gives a number to call in their SMS's, but when Clientele tried to contact them they never answered. Clientele then wrote a letter to them on the 15th of June 2018, but to date they did not respond to the letter, but have sent me a total of seven SMS's instead till yesterday. I am threatened with the sheriff, more legal action and blacklisting. I am 70 years old with no fixed income, but am expected to pay an alleged 18 year old debt, of nearly R52000, I did not even knew existed. Last year on Black Friday I was offered a 50% discount. Now they are offering a 30% discount. I wish they would tell me where I can find a job where I can earn enough to get them of my back, or at least answer the letter from Clientele.