This is currently possible by manually deleting/unsubscribing customers in your account. Here's how:
Add, edit, and delete customers
Unsubscribing or resubscribing customers
There isn't a way yet to bulk delete and unsubscribe users so if you need to delete/unsubscribe a lot of your customers, just let us know. :)
You can access our API documentation from here: Simplesat API