Garnishee order

I had a cell C contract from 2008 till 2010. My account was paid on monthly basis via debit order, hence I could have not missed payments. I paid off my account and Cell C/ Maravedi carried on debiting my account, that's when I contact them in order to get the payment reversed. In 2012 a sheriff was sent to my workplace which is a total disrespect and an embarrassment. I have already tried contacting them to sort out the whole issue. I cannot get hold of them and all their contact details are not going through. However, they still keep on harassing me to an extend of them sending me a garnishee order via payroll at my work. Can someone please sort this out, its the only means that can help me at this stage.