Hi Caroline,
There isn't an option to permanently delete customers from the QuickBooks Online (QBO) mobile app. However, I can offer you an alternative to achieve your goal.
To remove a customer, you can mark them as inactive. This will hide them from lists and menus, and you will need to use QBO through a web browser. Even if a customer is inactive, all their transactions will still be accessible in reports. Please note that if you delete a parent customer, all of their sub-customers will also be deleted.
To make a customer inactive using the QBO web browser, please follow these steps:
1. Sign in to your QBO account.
2. Go to the left panel, select Accounting, then click on Customer Hub and choose Customers.
3. Select the customer's name to open their profile.
4. Click the dropdown arrow next to Edit.
5. Select Make inactive, and then confirm by clicking Yes, make inactive.

Please note that if you view a transaction for an inactive customer, it will display as 'Name (deleted).'
Let me know if you have any further questions about QBO. I'm here to help!