A subscription can be terminated in the Admin Console or the API. The Hosted Account Management pages do not provide a customer facing terminate option.
To terminate a subscription in the Admin Console, find the account and click on the subscription name. In the Subscription Actions dropdown at the top right of the page, select Cancel Subscription. On the next page, click Terminate Subscription.
If the last invoice for the subscription was paid, you will see the option to refund all or a prorated amount of the last invoice. If the subscription includes usage-based add-ons with unbilled usage, you will see the option to not charge the customer for their final usage. Note that a final invoice is always sent if there are usage-based add-ons in the subscription. Choosing to not charge simply reduces the usage shown on the invoice to zero.