I've linked a customer in ServiceBox to a customer in accounting by accident, is there a way to remove this link?

Yes there is! Just go edit the customer (or jobsite) in ServiceBox. At the bottom of the screen will be a checkbox indicating that the customer is linked to a customer in your accounting software. Just uncheck the checkbox and click 'OK'. This will remove the link and the customer can be re-imported as a new customer or linked to an existing customer.