When all mitigation measures have been exhausted and the customer still wants to cancel:
1. Send following email to customer from Service Desk
...
Thanks for your note. We just cancelled your subscription and you will not be charged any further. You will have access to your account until [last subscription day].
We’re sad sorry to see you go but please reach out to us if you’d like to reactivate your account at any time.
...
Cancel subscription in Invoiced right after so that customer is not charged automatically the following month.
3.
...
Mark opportunity as cancelled in SFDC
And add reason, comment, date, etc.