I think there is probably an augment to be made and with in there T&C that you are in an implied contact which is legal in SA, as they are providing a monthly on going service.
I would also say with out reading there T&Cs they probably put the onuses on you to report if the service is still active after cancelling and you are liable for it should it still be active,
the below dives a little bit in to it.
When entering a contract, one of the most common misperceptions that the contracting parties have is that the contractual relationship is only governed by the terms as it appears in the particular contract.
www.duvenage.co.za
EDIT - I browsed the T&Cs
They pretty much can do this.
1 - They have a pretty broad definition of fees and can collect with out notice.
- 1.15.rain reserves the right at any time to collect any/all outstanding amount/s duewith respect to your Service (or any part or content thereof) without notice due to non-payment of subscription charges and other fees on your account(s).
2 - they put the onus on you to keep up with changes to T&Cs and can change them at any time after you accepted them
- 2.6.You acknowledge and understand that rain, as provider of the Service, may amend or change these Terms and Conditionsfrom time-to-time. Where rain changes the Terms and Conditions, the amended version will be made available on the rain Website. You agree to check the rain Website regularly for any amended version of the Terms and Conditions.
2.7.By continuing to use the rain Services after these Terms and Conditions have been amended, you acknowledge and understand that you have accepted and agreed to the relevant amendments or changes