Cannot Delete Key from System Keychain

I am having a bit of a problem deleting keys (any keys) from the System keychain on my iMac. Here's what happens:

1. I go to Keychain Access and select System from the list of keychains.
2. I unlock the keychain using my admin password (the icon changes from locked to unlocked).
3. I left-click on the key that I want to delete and select Delete "The Key I want to delete"
4. I get a box that says "Keychain Access is trying to modify the system keychain, Type your password to allow this." So I do.
5. I get a box that says "Are you sure you want to delete "The Key I want to delete" from the System keychain?" I click 'Delete'
6. I get a box that says "An error occurred while deleting "The Key I want to delete" UNIX[Operation not permitted]

I am not able to change any of the information in any of the System keys nor am I able to add new keys to the System keychain. My System keychain contains 20 items - all of which are either passwords to WiFis, passwords to Time Machines (which I want to delete), and one application password to the guest account. This being said, I would have no problem "resetting" the System keychain to default (empty) condition.

I also have a MacBook, which is not exhibiting this problem. That is to say that I can freely edit, delete and create keys in the System keychain. I followed the instructions here to backup the system keychain from my MacBook and install it on the iMac, but get an error saying "Operation not permitted".

Any ideas?

Thanks!