Sorry Ben, but goodybags can't be cancelled. Goodybags last for the full month, and new ones can not be started until the expiry date of the current one.
If you exceed your data allowance, you need airtime credit and will be charged at 2p/MB until the expiry date of the goodybag.
In the meantime, it's worth avoiding streaming media such as YouTube etc, and downloading the opera mini browser can save a significant amount of data traffic. This can greatly reduce your costs.