I was in a similar situation but worried long before the time, so I ran around trying to find alternatives.
Officially, I think once the contract runs out, they keep debiting you on a monthly basis (one month at a time) until you cancel. But you MUST cancel within a week of expiry (which would be today).
808 is a dead end a lot of the time, but I was fortunate to get through and do my cancellation last Monday morning - and everything worked out fine after that. In case you can't get through to that number, try 809, or 1761 (and ask for cancellation / retentions department).
If that fails, you can go into an MTN Service Centre and ask them to cancel the renewal / monthly debit order. Take your ID book and, just in case, a proof of address document. But you must go to a "Service Centre" - the MTN stores usually in big malls (in Cape Town, Cavendish, Canal Walk, Waterfront, N1 City)...NOT a franchise branch (which are a lot more common).
In all cases, make sure that if you want to stay on prepaid, TELL THEM! You don't want to lose your MTN number over a silly ommission.
Hope it works out for you