cancell service

hi i cancelled my service on the 8-8-2015 0n number *** when i enquirer a month later if it was done i got a bill for R736-50 they claim it was over usage of R500 that was activated i did not ask for it my understandig was that when i cancelled the number that it will be blocked they did not block it and my ex fiance used the device had they blocked the number the very same day no one would have used the number now i must pay for telkoms incompetence i think it is not fare i will move my number *** to the competition they just lost a loyal customer that they are *****ing