Interest keep on accumulating though I paid the full settlement amount.

M not happy with the interest that keeps on adding on my account while I paid the amount in full. I paid the full settlement amount at the end of November but the statement still came with R3 due for end of December. During the month of December I paid R10 but still when I log in to my account there is an amount of R0, 84 due against my account. I wrote an email 7 days ago requesting a paid up letter but they failed to help me.