Cancelling subscriptions can mean different things for renewing and non-renewing subscriptions.
How to cancel subscriptions
Publishers can cancel subscriptions by using the "Cancel" link on a subscription's page in your account.
Subscribers can cancel subscriptions from within their accounts.
Cancelling a renewing subscription will stop all future payments. Any paid-for, unsent issues will stay "active" as the subscriber has already paid for them. The subscription will be marked as "Active - Renewals stopped". Unsent issues will still show up in your exports.
If you want to cancel unsent issues, there is a toggle when cancelling within your Subsail publisher account (see below). This will mark the subscription as "Cancelled". You may want to offer a refund in this case as the subscriber has paid for issues they will never besent.
Cancelling a non-renewing subscription will cancel any unsent issues. The subscription will be marked as "Cancelled".