What you can do, as you may have been told by our AEs on the phone, is to apply a lock to the cRIO Remote System from MAX. Just open MAX>>Remote Systems>>[cRIOdevice]. In the Right pane of the MAX window, you have a "Lock" button at the top. Click it and insert a password. This password is applied to the system and anyone accessing the cRIO IP via ftp must provide username= guest and password = your_pwd, to access the cRIO.
Hope this helps,
AlessioD
National Instruments