You cannot delete an employee from the database when the record was used in any part of the system but you can make the employee inactive, disabling the timesheet and/or the login, prevent either or both actions.
Open the Employee Details page.
Go to the Main tab.
At the right hand side, check the box to Disable Login and/or Disable Timesheet as required.
Click OK to close the Employee Details page.
The user can only disable timesheet or login if he is allocated to the security group which allow those tasks.