Currently, the option to delete the customer permanently is unavailable. As a workaround, you can make it inactive. If you don't want to make the inactive customer show in the customer list or report, you can make a selection not to include it.
I've added some screenshots for more details on how to make the inactive customer show or hide in the customer list and on the reports.
I've included this article for your future reference in case you want to merge your customers: Add and manage customers in QuickBooks Online. This also contains other things on how to make your customer organized such as editing and restoring the inactive customer.
I'll be here to help you if you need anything with QuickBooks. Let me know in the comment section. Take care.