Harassment...!

***********... last year we made an arrangement for me to pay their client R395 that I was owing.. they sent me their bank details and a reference number. I did an ATM direct deposit of R400. The Atm did not issue a receipt, but it was a successful deposit. Till today they are still calling me and emailing me harassing me about the same payment. I don’t know what’s so difficult for them to track my payment with the reference they gave me.. Just because I can’t send them the slip that I made a payment in their books I still owe! I sick and tired of their phone calls. Please assist