Open the customer and use the “Delete this customer” panel at the bottom (administrators only). This is a full, permanent wipe: it removes the customer and every job and quote linked to them, along with all attached estimates, financials, expenses, hours, payments, photos, receipts and invoices. It cannot be undone, so take a database backup first if the customer has real history.
Use this for a test entry, a duplicate, or a record you are certain you will never need again.
If a customer has asked you to remove their personal information, use Tools, then Erase Personal Data instead. That clears their details without destroying the financial records you are required to keep. See “How do I remove a customer’s data on request?”