Unable to remove item from Time Machine Exclude list - switch to user account

If you click the Options button in Time Machine preferences you can specify items to exclude from backup. I’ve found it can be impossible to remove those excluded items; even deleting com.apple.TImeMachine plist (preference) files didn’t remove the item. It seemed to go away, but on return it was back.

It appears to be a permissions problem in Mavericks. I’d added the exclusion from a non-admin account, and I was working on Time Machine preferences from my admin account. I had to switch to the non-admin account to remove the item. I don’t know how to fix this if the originating account no longer exists. I don’t think Time Machine is storing this string in its preference file.