MBD Debt Collectors Harrasment

We paid up my husband Woolworths account more than 12 months ago. Suddenly we getting harassment calls 7:00 in the morning to pay the outstanding bill which is settled as per my knowledge we even have a paid up letter to confirm the account is fully settled. MBD Harrassing us on a daily basis about this. Getting sms with reference Dear MNR KRUGER kindly contact MBD LC on *** regarding your bought WOOLWORTHS acc. ***. The more you sent them emails confirm the account is fully settled the more the call you outside business hours every morning 3 or 4 times.