The option to delete customers is unavailable in QuickBooks Online. You can only make them inactive or changed their names.
Here's how:
Click Sales in the left panel.
Go to the Customers tab.
Select the customer's name you want to delete or change.
Hit Edit or Make inactive.
To help you manage your customers, you can refer to the following resources below. These will provide you steps on how to merge the same customers as well as add a sub-customer: