Using the subscription.canceled webhook, you can have Bizplay send a cancellation request to your administration system. You do have to make sure that the subscription's External reference in Bizplay contains the ID of the corresponding record in your administration system. It is assumed that your administration system will take care of the subsequent invoicing.
If you have the subscription.canceled webhook defined, then your customer will have the Cancel subscription available on the subscription edit page. Otherwise, the customer will see a message on his company details page that he should contact you in order to cancel.
If you're not using the subscription.canceled webhook, then you need to make sure that you cancel the subscription both in your administration system and in Bizplay manually.