Step by step
Prefer Edit first
If the person might return or you only need better contact data, open Edit instead of Delete.
Choose Delete
On Customers, open row actions and choose Delete.
Confirm
Confirm the browser dialog. Product copy asks whether you are sure you want to delete this customer (EN and NB strings exist in i18n).
What happens
The customer is hard-deleted from the registry. There is no Restore on Customers. Existing bookings remain in Calendar but their customer link is cleared (customer_id set to null). Customer packages linked to this person are removed with the customer.