To close access to the system or delete an employee you need to:

1. Go to Settings → Employees.
2. Select an employee.
3. Press the "Delete" icon.
4. After the account has been deleted the employee will be revoked the access to the system and the account will be stored in the archive (will be marked in gray)
5. You can restore the access to the deleted account. To do that choose the account and press the "Restore" icon.