cellphone is on prepaid but still charge on my acc

In Aug '15 Vodacom fetched their handset from me due to no signal and data going missing. The existing contract will then be cancelled. 2 months down the line and they still billing me for that handset even though the cell number shows as a prepaid number. No one at Vodacom seems to understand or to get this right. Prepaid means in order to use your handset you need to load airtime or data NOT bill me at the end of the month.<br>