Some SAIX resellers do allow you to authenticate once capped, but Telkom does not. IIRC, there's a guest Telkom account (don't know the details) that is never capped but you can only surf Telkom's site using it. If you can find the account details, see if you can connect with it. If you can't (and you're sure the details are correct) then your problem isn't with your account; phone Telkom.
What about Telkoms ISP? Would they allow me to authenticate? Also, wasn't there a Telkom site that you can still visit even if you're capped, if they are your ISP?
What's your usage?
The details for the guest account are:
User name: guest@telkomadsl
Password: guest