Unauthorised additional charges

Took out a contract with Telkom in 2016. The deal included a free router and 30GB day + 30GB night data for R499. To my absolute horror, they started debiting R950. Checked my bill, and there's an extra R500 worth of charges added to it. No notification, no explanation. Just straight up theft. In true Telkom fashion, they're slow to respond. I just want to cancel my subscription. Due for an upgrade, but there's no way I'll keep paying a company as *********** as Telkom. CANCEL YOUR SUBSCRIPTION NOW!