You can delete customer data using the web UI or via API as long as certain prerequisites are fulfilled. In the web UI, go to the customer list and click on the "Delete" button and confirm the deletion with "Yes". You can use DELETE /Customers/:CustomerId via API.
To be able to delete a customer, an invoice must not have been created for this customer. In this way, you can completely delete customers who do not yet have a contract or customers whose free trial phase has expired and who are now in the status "Trial Ended".