As long as they have their payment method set to not renew, then the subscription will end on the END date of that subscription.
Just as Ruben advised, all the user needs to do is cancel the subscription payment through their PayPal account.
At the end of the UBBT subscription, UBBT will attempt to contact PayPal and request another payment for that "Subscription ID." When it gets denied (because the user canceled the sub), UBBT will then close it out and the subscription will be seen as ended.
The user gets 100% of their subscription time, no matter if they canceled early or in the middle of the subscription, because 100% of the subscription is paid in full at the start date.
If you've talked with your user and there needs to be a custom arrangement, it can be managed through Control Panel > Transactions, all manually.
