TOP UP CONTRACT CHANGED INTO OPEN LINE

I HAVE ALWAYS BEEN ON A TOP UP CONTRACTS WITH VODACOM. WHEN MY DEBIT ORDER WENT OFF I GOT THE SHOCK OF A LIFETIME. R1700. WHEN I PHONED VODACOM THEY ADVISED THAT MY WIFE'S PHONE IS AN OPEN LINE AND SHE CAN CALL AS MUCH AS SHE LIKES. HER ACC IS R900 WHICH IS SUPPOSED TO BE R200. I HAVE NEVER REQUESTED THE LINE TO BE OPENED. WHAT CAN I DO?