According to this Telkom implements hardcapping on ADSL accounts:
From the Business ADSL 1024K FAQs:
http://www.telkom.co.za/attheoffice/products/dsl/business_faq.html
How does the DSL 3GByte monthly volume cap work?
The 3Gbyte Internet service available from most Internet Service Providers (ISP's) measures your local, international up and download usage and once the 3Gbyte cap is reached your DSL access will terminate which means you will no longer be able to access the Internet on an DSL connection until the reset at the beginning of the following month or until you purchase an additional username from your preferred ISP.
From the Business ADSL 1024K FAQs:
http://www.telkom.co.za/attheoffice/products/dsl/business_faq.html