Hi,
You have to adjust the existing access rights in the db such a way that a new user group will be introduced and that group will have only read, write and create permission. Instead of above approach is you can raise  validation error from the code side by inheriting the unlink method or by using an automated action from the db.

* Create a group, lets say "Block Employee Deletion"
* From unlink method check if user is part of the group and if yes, raise validation error

OR

* From automated action, which triggers on deletion, do the same checking and raise error

Thanks