Person record does not seem to be purged completely

Summary: Person record does not seem to be purged completely.

Content (please ensure you mask any confidential information): We created 3 dummy employees in our test pod to do some initial testing. Now we need to purge these dummy records because we are about to load the client's data. And as luck has it the client's data contain those exact person numbers that we created for the dummy employees. We thought we purged the 3 dummy records, but when we tried to load the client's data we received error messages referring to the existing of the dummy employees.

Any ideas how to resolve this?