SCS(PTY)Ltd STOP PESTERING ME!

SCS(PTY)Ltd Debt Collectors has been harassing me for months regarding an account which is'nt mine. Their ENDLESS phone calls and sms's are driving me crazy. The account is obviously **********. Today they stepped up their threats by giving me an ultimatum to pay in 10 days time. I checked AGAIN with the company they're supposed to collect the debt for- they know nothing about it. How do I get these people to stop harassing me?