Depending on which data tariff you have subscribed to, you need to send a text with "STOP DATENFLAT2" for the Internet Pack L (500MB) or "STOP DATENFLAT" for the Internet Pack XL (5GB) to 7777.
If your remaining balance is lower than the cost of extending your data option for another month your subscription to whichever data option will be cancelled automatically.
Additional information is available at
Germany - Tchibo - Prepaid Wireless Internet Access