Being billed against a CREDIT bal being settlement

Cancelled contract in Nov 14. Paid full settlement figure R2152+ & sent confirmation to cell c.They continue billing me against the CREDIT BALANCE!After several complaints lodged was advised contract cancelled end Jan 15. Still being debited for R2152+ to my bank account every month and receive calls monthly from collections dept.asking when I am going to settle. A different agent calls every month and then after another frustrating conversation where I explain what happens they promise to investigate & revert but never resolve and give feedback.