Trying to collect a debt that does not exist

For some time now i have been receiving threatening SMS messages from this company. I have contacted them and told them this debt does not exist and to either send me details of the account or a letter confirming this debt is clear. I have an email chain which go unanswered. I have sent the email more than 10 times with only one reply. The emails go to Thandi and Gerda. This debt is obviously on a \list"that NB have bought. We had the same problem with another collection agency some time ago and had to revert to paying for a legal firm to send them letters. I have forward the same letter to NB. IMO they are just taking chances on collecting debt from Bought Lists. How many people just pay when they are threatened with blacklisting etc. This must be bordering on being *******. Next step now is the Ombudsman. Account 1AB003880"""