You do not mess with "system" permission.
Try these:
. Restart your Mac holding down "Command(the Apple key)+S" key to enter 'Single user mode'.
. Type "fsck -f", and hit return.
. Once complete type "reboot" and hit return to reboot you Mac back in to OS X as normal.

If you have another Mac, boot into target disk mode and use another Mac to do Repair Permission.

If you want to avoid access from other client in the network, why don't you just disable your File Sharing?

lol. yeah, have to learn the hard way.