Harassing Phone Calls

Midland Credit keeps calling be about 8AM (waking me up) and has done do for the past several days. They ask for my daughter Lynne who does not live at my address here and has never lived at my address. When I explain this to them they just hang up. The next morning I get the same call again. I phoned them today and, while they agree to remove my phone number, they were so nasty that I don't believe them. Today, I also sent them a certified letter asking them to cease. What is my next step if they phone me again?
John