In QuickBooks Online, the option to delete a customer is by making them inactive. Here's what you'll need to do:
In the left navigation menu, click Sales.
Go to the Customers tab.
Put a check mark on the box beside the customer's name.
Click the Batch actions drop-down, then choose Make inactive.
Select Yes to confirm the deletion.
Keep in mind that when you delete a customer, the transactions associated with them, remain on your company data in Reports. Also, when deleting a parent customer, the sub-customers will also be deleted.