If you've booked a customer into the wrong class series here's how to cancel them out.
Locate the next class they are registered for that shows them listed as 'unpaid'. (click Classes > Schedule). To the right of their name click the dropdown menu > choose 'Cancel Recurring booking'.
This will cancel them out of the remaining this class plus the remaining series. If there is an initial class that they were booked into and have already paid for you'll need to cancel them out of that one using the normal cancel option, available in the same dropdown menu.