You would have to have login capabilities, because you would have to keep track of who it was that was unsubscribing. You would store their paypal profile id associated with the recurring payment, then you could add a cancel subscription button that uses the profile id to cancel it using the paypal API.
You would need to own both Data Bridge (for login) and eCart (for subscription cancel).
If you are using the free paypal buttons, the only easy option is to tell people to cancel their subscriptions from their paypal profile directly.
