Spamming from Finchoice

Had 6 month loan with Finchoice, paid by debit order every month. on the last month I changed banking details and made a cash payment a day after the debit order would have been due (25 instead of 26 the of the month), and this supposedly attracted interest. I am being harrassed daily by no more less than 10 collection calls for R100 owed. Explaining to the call centre agents is futile as I continue to receive more calls each day.