The rule is that you cannot merge an employee into a patron. The reason is that every record in the system it tagged with one employee's name or another for the various entered by, changed by, sold by, printed by (etc) fields -- and deleting the employee would leave a referential integrity problem -- records would have missing data.
As mentioned above, you can't simply delete the employee record to make them a non-employee.
Normally you would edit their employee record and set a resigned date. This means they can no longer log in while preserving a history of what they changed in transactions - as required for PCI compliance.
However, should you really need to remove an employee record created in error (or who resigned decades ago), there is a simple trick you can use:
Effectively you are just assigning all the changed by information from these employees to be yourself. Then it removes person as an employee.
Once they are no longer employees, you can merge the patron records.