Harassment for Prescribed Debt

I am inundated with emails and sms messages almost daily from NuDebt collection agency in behalf of standard bank for a credit card debt which has prescribed. They claimed I made a payment in 2014 and when I asked for proof they sent a letterhead from NuDebt stating I paid. I then requested a receipt or statement of some form to see how the apparent payment was made because I certainly had no knowledge of it whatsoever....A letter from an admin person at NuDebt does not satisfy this requirement. No such proof has been forthcoming and the constant string of sms and email messages continue on.Sometimes in the late hours of the night! Standard bank is supposed to be a large institution well aware of prescription laws and should know better than to ask an agency to harass a former debtor *******ly. I would appreciate it if they get someone to stop the abuse and ensure the matter is dealt with swiftly.