Vodacom has implemented a blocking mechanism for all OBS transactions and created an error code for this purpose. This was communicated to all WASPs on 21 October 2010. From 1 December 2010 any billing attempt that returns error code 41 will represent "subscriber locked for content billing". Don't expect this feature to be marketed widely.
This is now required by law - although it is badly worded: Gvt Gazette (Notice 546 of 2010) : "(3) A provider must, free of charge to an end-user, block the origination of premium rate communications from a number assigned to the end-user if the end-user has requested the blocking. "