Annoyed with *********** staff

MBD Credit Solutions has been harassing me since beginning December to pay them money that I don't owe. I went for blood tests last year, which came to an amount just over R2500, received a statement from the National Health laboratory for an amount of R159,30 which I paid on the 30th of November 2017, the medical aid paid the rest of the bill. Since December I have received numerous sms's and calls relating to the account, as much as I try to explain to each consultant that the amount they are attempting to claim from me has been settled it seems that either they are *********** at doing their job or they're hoping that I will just pay the money to get rid of them. I've sent through the medical aid statement, the invoice from the NHL as well as the proof of payment but to no avail. Everyday I am harassed by their consultants and when I get upset they just put phone down instead of assisting me. Seriously Frustrated and ****ed Off!!!