Continuous harrassment on prescribed debt

I am getting continuous calls (up to 6 times a day starting at 6am in the morning and sometimes quite threatening) from DMC about prescribed debt. I have told numerous call centre agents that the debts they keep phoning about has been prescribed and that I am under no obligation to make any arrangements with them. They have started phoning at work when i have made it clear to a guy named Marlon that I cannot take personal calls at work. If this continues I will lay a charge of harrassment with the Law Society. What DMC is doing is ********* in every sense and needs to stop!