ReadyNas Duo & ReadyNas RN104

You will need to change server settings for this.  Share access depends on the user credentials presented to the NAS.  Windows presents a user logon/password to the NAS, and the NAS then applies whatever access limits apply to that user.

Windows only allows one user logon for a remote device at a time (though it does treat the IP address and device name as two separate devices).  So if you have a share open in windows, you can't specify a different password for another share in the left pane.

Windows will also store user credentials for future use (e.g., ask you if you want it to remember the username/password).  And if anyone using the computer tells it to do that on the blocked shares, then future users will be able to see the blocked shares (until you delete the credentials manually from the windows credential manager).

What you could perhaps do is create two user accounts on the PC - one which has full access to the NAS, and another that has partial access.  That would require switching accounts whenever you wanted to move to the other access mode.  (And the partial access would need to be set up on the NAS).

You can also set up hidden shares.  These are accessible if you know how, but not visible. 

EDIT - thinking a bit more, there is something you can do in Windows if you are willing to block all access to all shares (and don't use a shared printer).  You can set the windows sharing controls to turn off file and printer sharing, and then simply turn on file and printer sharing again when you want access.  You might be able to find internet securty software that does something similar.