When a customer wants to cancel their recurring payment you can look up their Pre-Approval number (it begins with "PA-") by going to Sellers->Reports->Preapprovals. From there you can cancel any future charges for the customer. You can do the same thing by looking up the payment ID in the transactions report.
You can also cancel your subscriptions and preapprovals by going to your Paypal account directly.
In order to cancel your subscriptions, you will need to go to your account settings, then to subscriptions. The direct link is: https://www.paypal.com/myaccount/settings
If you are trying to cancel a pre-approval, you will not find that in your list of subscriptions. You will need to be logged into Paypal and go directly to:
That will allow you to control, and more importantly cancel, any preapprovals that you have setup with Paypal.